Eight inbred lines of summer squash were crossed in line x tester method. The parental lines [6 lines (female) + 2 Testers (male)] and their off springs (12 hybrids) were planted out in the Vegetable Field of Horticulture Department, College of Agriculture, Abu-Ghraib during the spring season of 2002 in RCBD. The experiment aimed to evaluate the yield of hybrid and their parents, beside the N, P, K content in the leaves, and to estimated heterosis and genetic parameters in some characteristics. The results showed the superiority of the two inbred lines (T1 and L8) in N concentration in leaves, number of nodes in the branches and leaf area. Inbred lines T2 and L5 had higher values of number of flower set and plant yield. However, the inbred lines (female) showed a superiority in P concentration in leaves as compared to the mean. The hybrid (L8 x T1) had higher values of N concentration in leaves, number of nodes in the branches and leaf area, while the hybrid L8 x T1 showed the higher percentage of hybrid vigour in N%. The hybrids (L4 x T1), (L7 x T1) and (L7 x T2) had higher values and the higher hybrid vigour in plant yield (3.1, 2.55 and 2.28 kg / plant respectively). These hybrids regarded as promising hybrids. The inbred lines L8 (with N concentration in leaves, number of node in the branches and leaves area) L4 and L7 (with number of flower set and plant yield) showed GCA effect in preferable trends. There parents are good combiner and could be used to improve some characters in squash. Two hybrids (L4 x T1) and (L8 x T2) had the best SCA effect in plant yield. The results showed greater 2sca than 2gca in plant yield , leaf N content, and number of nodes in the branches, Therefore these traits were effected by over – dominance gene effect. Narrow – sense heritability (h2n.s%) was low while broad – sense heritability (h2b.s%) was moderately high which could be used in hybridization method to improve plant yield and N content. The lines (female) had a high contribution to total variance for studied traits . The line x testers had a good contribution in total variance, while the testers were of low contribution to total variance for some traits.