Watermelon (citrullus lanatus. Thunb.) is an important vegetable crop grown in Egypt. This study was planned to obtain an information about the nature of heterosis , gene action and correlation among yield and yield component traits of water melon.These genetic information could be used to improve the productivity and quality of watermelon . A complete diallel crosses mating design was used among five watermelon varieties named as; Giza-1, Giza- 21, Charleston gray, Crimson sweet and Dulzero were performed. Different  traits were number of fruit per plant ,total yield per plant in kilogram,total yield per plot in kilogram, average fruit weight ,fruit length , fruit daimeter and shape index. The results reveald that the average means of the F₁ hybrids, F_1r reciprocal hybrids and overall F1 hybrids significantly exceeded the mid-parents , althought there was no single hybrids exceeded the means of the parental varieties for all studied yield and yield component traits . The results also indicated that the magnitudes of the non-additive genetic variances including dominance were larger than their corresponding estimates of additive genetic variances for all studied traits.In the same time , the obtained values of additive genetic variances could not be neglected The estimates of heritability in broad sense were larger in magnitudes than those  estimates of heritability in narrow sense for all studied traits. The results illustrated that both phenotypic and genotypic correlation coefficients values were closed to each other with respect to most pairs of studied traits. In this respect ,positive genotypic (rg) and phenotypic (rph) correlation and highly significant was present.It could be regarded that(N.F./P.) was significantly correlated (T.Y/P..), (T.Y/Pt.), (A.F.W kg),  In general ,selection program specially reciprocal recurrent selection could be used to improve watermelon traits .