A full diallel cross-based on four yellow maize inbred lines was performed during 1996 summer season. During 1997 season all entries 16 genotypes (6F₁'s + 6F₁ reciprocals + 4 parents lines) were evaluated at two locations; Sids and Giza. The aim of this study is to define; crosses mean performance and the potentiality of heterosis expression, the magnitude of general, specific combining abilities and reciprocal effects, components of genetic variation and type of gene action for some morphological, yield and yield components. Data confirmed that variance due to SCA was more important than GCA. Maternal effect was noted in the inheritance of plant height, stem diameter, number of rows per ear. The parental D₁ line was found to be the best based on general combining ability. Two crosses namely, D₂ X L₂ were on the top of other crosses. The dominance component (H₁) and (H₂) were found to be significant at both locations, whereas additive effect of inheritance was negligible. While, both dominance and additive genetic effects controlled the inheritance of number of leaves/plant, Moreover, maternal effects were also involved in the inheritance of this trait, as indicated by L₁ X D₁ cross. L2 X D2 and L2 X L1 manifested desirable heterosis.However,ear height showed,dominance components with greater values than additive genetic variance. The common parent in these crosses was L₂ line that showed the highest general combining ability for such trait. Total grain yield / plant and number of ears/plant proved that, environmental conditions played major role in the inheritance of this character. However, dominant gene action was highly significant, whereas additive gene action had no detected role in controlling this trait. Maternal effect was clear in L₁xD₁ cross. D₁ line was a good general combinor, and heterosis was clearly detectable in cross D₁x L₁ and D₂xL₂.