Six inbred lines of summer squash representing wide range of variability in most of the studied traits were utilized in a half diallel cross. The studied traits were  Plant height, days to female flowers, number of fruits/plant, average fruit length, average fruit diameter, average fruit weight, early yield/plant and total yield/plant. Heterotic performance, graphically variance/covariance analysis and genetic components were used to analyze the obtained data. Results showed that the maximum significant true heterosis (BP) in desirable direction (179.9%) was recorded for early yield/plant followed by total yield (106.9%), fruits number/plant(57.0%), plant height (40.9%), average fruit weight (32.5%) and days to female flowering date (-17.2%). Data of diallel table for each trait were subjected to graphical analysis to obtain information on the adequacy of the additive-dominance model of gene action. The relative position of array points on Vr/Wr graph indicated that parental genotype P₁ (line-280) contained high frequency of dominant alleles for earliness, fruit length, fruit diameter, fruits number and both early and total yield. Results of diallel analysis indicated that the value (H/D)^1/2  was more than unity for all studied traits indicating the presence of over-dominance and supporting the graphical conclusions. However, the predominance of non-additive gene effects and values of h_n.s for most important traits suggesting the possibility exploiting dominance gene effects for improving such traits through heterosis breeding.