Half diallel set of crosses involving six durum wheat parental lines were used to estimate the type and relative amount of genetic variance components for yield and its attributes under two locations. The results revealed that additive type of gene action was found to reach the level of significance in some cases at the two locations. Dominance gene effects were larger in magnitude than the additive ones and appeared to be the more prevailing for all the studied traits under both locations. "F" value coupled with Ko/Kr showed an excess of positive and negative alleles in parents at the two locations. Studies on the nature and degree of dominance revealed the existence of over-dominance for all studied traits at the two locations. The correlation between the parental performance and the parental order of dominance were found to be negative for all studied traits studied except plant height and number of grains per ear at the two locations and for ear length and number of spikelets per ear at the second location. High estimates of heritability in the broad sense were accompanied by moderate to low estimates for the narrow sense were detected for most traits.