Abstract: A 6 × 6 diallel cross consisting of six wheat varieties/lines namely Pak 81, LU26S, Inqlab 91, Rohtas 90, 4072 and 4943 was analyzed to determine the nature of genetic mechanisms controlling some Morpho- physiological traits, like stomatal frequency, leaf venation, flag leaf area, specific flag leaf weight, days to heading, tillers per plant, plant height, spike length, grains per spike, 1000-grain weight, biomass per plant and grain yield per plant under irrigated and drought stress conditions. Adequacy test satisfied the simple additive dominance model. High estimates of narrow sense heritability were found for stomata! frequency, specific flag leaf weight, days to heading, plant height, spike length and 1000-grain weight, also observed lower average degree of dominance under both environments. In the present study additive genetic effects were found to be more important than nonadditive effects with the exception of yield per plant which appeared to be more closely linked to nonadditive genetic effects. Since the yield components like grains per spike, 1000-grain weight, tillers per plant were found to be significantly influenced by additive gene action, further progress in genetic improvement of the yielding ability of wheat varieties may be attempted through such yield-related characters, with much better prospects of fixing desirable genes in a single homozygote.