Global warming due to fossil fuel emissions, depletion of petroleum product reserves, population growth and crude oil price hikes have stimulated researchers to search for economical and environment friendly alternative energy resources. This study firstly reports the development of a novel, cheap, easy and environmental friendly regeneration method of waste automotive oil. Secondly, efforts were made to convert Pakistani waste oils such as waste engine oil, waste transformer oil, waste tyre pyrolysis oil and waste cooking oil into biodiesel and biodiesel-diesel fuel blends to be used in diesel engines without any engine modification. A new method was developed to regenerate waste automotive engine oils into valuable base oil using cheaply available Rice Husk Ash (RHA) as raw material.Optimum results were observed only with 6% activated RHA. The advantage of using an activated rice husk ash is that it does not react with base oils and involves use of only 1% acetic acid as compared to conventional acid clay method involving the use of 6-8% Conc. H2SO4 which is harmful to the environment. Another advantage of using acetic acid is that it does not emit poisonous gases like sulfur dioxide to the atmosphere. Efforts were made to prepare biodiesel like fuel blends derived from pretreated waste engine oil, waste transformer oil, waste tyre pyrolysis oil and waste cooking oil. Experimental investigations on these Pakistani waste oils were carried out in a 5.5kW four-stroke single-cylinder water cooled direct-injection diesel engine for combustion, performance and emission characteristics. Results obtained were compared with those of petroleum diesel. All the fuel blends run in the diesel engine have shown slightly higher fuel consumptions and shorter ignition delays in comparison to petroleum diesel. The engine was successfully operated with waste oil derived biodiesel fuel blends without engine failure but the best performance was observed with CFB10, vii BLF15, BLF20, FMWO10, WCOB10 and WCOB15 biodiesel fuel blends. Biodiesel blends derived from regenerated waste engine oils have shown significantly lower HC, CO and smoke emissions as compared to petroleum based diesel. Biodiesel fuel blends derived from waste transformer oil have indicated lower HC, CO and smoke emissions as 10.92-31.17%, 3.80-6.32% and 1.39-5.21% respectively in comparison to those of petroleum diesel. Fuel blends derived from waste tyre pyrolysis oil have shown 3.12-15.62%, 16.5-33.2% and 1.83 - 4.5% lesser CO, HC and smoke emissions compared to petroleum diesel fuel. Similarly, biodiesel fuel blends derived from waste cooking oil have shown significantly reduced pollutant emissions. All the biodiesel-diesel fuel blends derived from waste oils mentioned above were characterized by FTIR and the results were compared with those of petroleum diesel. Like petroleum diesel, the obtained FTIR analysis results also confirmed the presence of saturated alkanes in the investigated biodiesel fuel blends. Biodiesels derived from regenerated waste oils and their blends were characterized for physico chemical properties using ASTM (American Society for Testing and Material) standard methods and the results were compared with petroleum diesel. Analysis results were found within the permissible limits of biodiesel fuel as specified by European (EN14214) and American (ASTM 6751) standards. Current research demonstrates the beneficial and environment friendly conversion of waste oils into useful commodity i.e. biodiesel to be used as alternative fuels for diesel engines. On the whole, this will reduce hazardous waste oil disposal problems, minimize environmental issues and boost the economy of Pakistan by minimizing its dependence on foreign origin crude oil reserves for mineral diesel fuel.