The seeds of one hundred maize inbred lines collected from various research organizations were planted in two sets, one under normal and the other under high temperature conditions in a plastic tunnel for the purpose of screening against heat at reproductive stage. Based on the actual and relative values for leaf temperature, cell membrane thermo-stability, stomatal conductance, transpiration rate, leaf firing, kernels per ear, 100-grain weight and grain yield per plant, one heat tolerant (ZL-11271) and one heat susceptible (R-2304-2) parents were selected and crossed to develop six basic generations comprised parents (P1, P2), hybrid (F1) and segregating generations (BC1, BC2, F2) in subsequent cropping seasons. All these generations were then evaluated under both normal (field) and heat-stressed (plastic tunnel) conditions using factorial randomized complete block design with three replications. The recorded data under both the conditions on various morphological and physiological plant traits were analyzed in nested block design for one way, two way and partitioned analysis of variances which revealed statistically significant differences (P≤0.01-0.05) for all the characters except anthesis-silking interval. Generation mean analysis of plant traits recorded under normal conditions revealed both additive and dominance genetic effects alongwith epistatic interactions for leaf temperature, cell membrane thermo-stability, stomatal conductance, leaf firing, plant height, ear leaf area, days to maturity, ear length, kernels per ear, 100-grain weight and grain yield per plant. For all these traits except stomatal conductance, dominance effects were more pronounced than additive estimates. Only additive genetic effects alongwith epistatic interactions were revealed for transpiration rate, days to tasseling and days to silking under normal conditions. Days to silking and days to maturity had dominance genetic effects with no epistatic interaction while traits like leaf temperature, cell membrane thermo-stability, stomatal conductance, transpiration rate, leaf firing, plant height, days to tasseling, ear leaf area, ear length, kernels per ear, 100-grain weight and grain yield per plant revealed both additive and dominance genetic effects alongwith epistatic interactions under heat-stressed conditions. Additive genetic effects were greater in magnitude for leaf temperature, cell membrane thermo-stability and stomatal conductance while estimates of dominance genetic effects were higher in case of transpiration rate, leaf firing, plant height, ear leaf area, ear length, kernels per ear, 100-grain weight and grain yield per plant under heat-stressed regime. Estimates of broad sense heritability were higher than that of narrow sense heritability while estimates of narrow sense heritability for infinity generation were greater than its F2 generation for all the traits. Considering the estimates of heritability and genetic advance at once suggested that only simple selections might be enough for further improvement of traits such as cell membrane thermo-stability, stomatal conductance, transpiration rate, leaf firing, ear length, kernels per ear and grain yield per plant under both the condition. Grain yield per plant had positive and significant association with stomatal conductance, transpiration rate, ear length and kernels per ear while negative but significant with leaf temperature, cell membrane thermo-stability, leaf firing and 100-grain weight at both genotypic and phenotypic levels under both normal and heat-stressed conditions. Ear leaf area exhibited positive and negative association only at genotypic level with grain yield under normal and heat-stressed conditions, respectively. It can be concluded that traits like cell membrane thermo-stability, ear leaf area and kernels per ear may be given priority in breeding strategies for achieving improvement in maize grain yield under high temperature circumstances.