The main objective of this investigation was to determine the genetic parameters, heritability in both broad and narrow senses, combining abilities and correlation foryield and yield component traitsin squash (Cucurbita pepo, L.). For this purpose, five squash varieties were used .These varieties were: Eskandrani (P₁); Zucchino tondo di piacenza (P₂); Black beouty (P₃); Zucchino romanesco (P₄) and Coppi (P₅). During the summer season of 2011, acomplete diallel crosses mating design was made to evaluate the performances of  the 20 F₁ hybrids including the reciprocal will their five  parental varieties in afield trial at the Experimental Station, Faculty of Agriculture, Mansoura University.       Data were recorded on the following traits, number of fruitsper plant (No.F./P.), fruits yield per plant (F.Y./P.Kg), fruit length (F.L.cm), fruit diameter (F.D.cm) and  fruit shape index (F.Sh.I).              The results indicated that the mean squares for all genotypes were highly significant for all yield and yield component traits.                    The results of means showed that Eskandrani (P₁) was the highest parent for Number of fruits per plant No.F./P.  the parental variety Zucchino romanesco (P₄)  was the highest variety for F.L.cm and F.Sh.I. However, the highest F₁ hybrid for fruit yield per plant was Eskandrani (P₁)  × Zucchino tondo di piacenza (P₂)with the mean value  of 11.24 kg. Whereas, the highest F_1r  hybrid was Zucchino romanesco (P₄)  × Eskandrani (P₁)  with the mean value of 10.51 kg.                 The results revealed that the GCA effects (gi) were highly significant for the two parents: Eskandrani (P₁)  and Zucchino romanesco (P₄) for F.L.cm, F.D.cm and F.Sh.I traits. At the same time, the results also revealed that the GCA effects were highly significant for the parents: Zucchino tondo di Piacenza (P₂) for F.L.cm and F.D.cm;  Black beouty (P₃) for F.D.cm and F.Sh.I and Coppi (P₅) for F.Sh.I.                 The results indicated the importance of both (GCA) and (SCA) combining abilities. General combining abilities (GCA) were larger than specific combining abilities (SCA) for all yield and yield component traits except fruits yield per plant (F.Y./P.Kg). The estimates of heritability in broad sense (h²_b) were larger in magnitudes than those of narrow sense (h²_n) for all studied traits. The values of heritabilities in broad sense ranged from0.513 to 0.894 for yield and yield component traits.