Over the past twenty years; road traffic (both passenger and freight) has grown significantly and loading is progressively getting worse due to the introduction of newer and more powerful trucks with heavier wider bodies in Pakistan. Consequently, premature rutting in the form of shear flow in flexible pavements has been observed during high ambient temperatures. National Highway Authority (NHA), Pakistan has been facing serious threats like, frequent pavement failures, poor riding quality and high maintenance cost. Modifications of Asphalt cement with polymers and rigid pavement design trends, being the end solutions, have increased the construction cost, even more than four times than that of conventional design. In order to cater for the growing axle load demand and to increase the performance of asphalt concrete mixes, true prediction and accurate estimation of probable behavior of mixes need to be investigated. A comprehensive laboratory study was carried out using NHA aggregate gradation Class “A” for asphaltic wearing course, which has commonly been used in the field. The main objective of the research work was to evaluate the effects of temperature and loading on the permanent deformation behavior of mixes, designed with the same aggregate gradation and three commonly available asphalt cement types (A.C). Two gradations i.e. “01” and “02”, within the envelope of the same gradation, were chosen for this study. Three asphalt cement (A.C.) types i.e. two neat A.C with penetration grade “60/70”, “40/50” and one modified A.C. (base “60/70” pen. grade with Elvaloy Terploymer) were selected. Six mixes ranging from finer to coarser aggregate gradation were therefore designed at optimum filler contents, in order to get better mix cohesion, resistant to rutting and to improve serviceability. Three percentages of mineral fillers (i.e. 2.4%, 3.4%. & 4.4%) were trialed in order to determine the optimum filler content for asphalt mixes. Mix design properties i.e. optimum asphalt content, percentage air voids, voids in mineral aggregates, voids filled with asphalt, flow and stability were determined using Marshall Method of Mix Design. Asphalt cement consistency i.e. penetration grade, ductility, softening point and rheological properties i.e. phase angle & complex shear modulus were IIIalso measured. The temperature influence on rheological properties were determined at constant frequency of 10 Hz using Dynamic Shear Rheometer. Two performance tests i.e. uniaxial repeated loading strain & wheel tracking tests were chosen to measure the permanent deformation of asphalt mixes. Relationship between rutting and factors effecting rutting were developed. Domains of intercept and slope coefficient and permanent deformation coefficients alpha (α) and mu (μ) were also determined. New relationships have been developed to predict the impact of Hot Mix Asphalt properties on its permanent deformation behaviour. It has been revealed that mixes with coarser gradation and polymer modified asphalt showed better resistance against rutting. Permanent deformation coefficients, such as α and μ indicate influence of mix properties on mix rut potential. It was further observed that temperature has no effect on α but influences significantly on μ. This study presents relationships developed to correlate temperature and rut values of mixes, tested under wheel tracker (WT). It also presents a comparison between uni-axial repeated load strain tests (or repeated creep test) and wheel tracker tests in terms of regression constants, shift factors and a correlation between both the test methods. Intercept coefficient varies in a narrow range. Based on intercept coefficient, uniaxial repeated load strain test (repeated creep test) did not provide clear ranking of mixes at specified temperatures and stress conditions. The slope of data line between both the tests reduces with increase in temperature and stress levels, which means permanent deformation increases with an increase in temperature and stress. An average value has been determined for approximate shift of creep test data to WT test data. Furthermore a statistical estimation of Mechanistic-Empirical Model has been formulated that relates plastic to elastic strain ratio with different variable that influenced in the mix rut development. Using 54 variables, comprising six mixes designed at low asphalt contents, three temperatures (25, 40 and 55 0 C) and three stress levels (100, 300 and 500 kPa) under repeated load test, a mathematical model was developed to assess the magnitude of plastic to elastic strain ratio. It was observed that plastic to elastic strain ratio is a function of number of load repetitions, temperature, stress levels, shear complex modulus of asphalt cement and IVaggregate gradations respectively. Despite few limitations i.e. single source of aggregate, single method for mix design, two test methods, certain parameters that help to estimate the permanent deformation in the asphalt layers of flexible pavements were successfully captured in the model. Keywords: Flexible Pavement, Hot Mix Asphalt, Uniaxial Repeated Loading, Wheel Tracker, Permanent Deformation