A half diallel set of crosses among six inbred lines of maize were evaluated at the Maize Research Department (G.C.S.A.R.) in Damascus Governorate during 2010 and 2011 growing seasons to estimate heterosis and combining ability components for the number of rows per ear, number of kernels per ear, ear length (cm), ear diameter(cm), 100-kernel weight (g), and grain yield (ton /hec). The inbred lines, crosses, general combining ability (GCA) and specific combining ability (SCA) mean squares were highly significant for all the studied traits, showing the existence of a genetic variance among lines. The ratios of GCA to SCA detected for the studied traits showed the dominance of an additive gene action for all the studied traits except for 100-kernel weight and grain yield which showed the predominance of a non-additive gene action. The heterosis percentage for the studied traits was significant based on mid and better parents. The GCA effects showed that the lines CML.317, CML.371, CML.373 and CML.367 were good general combiners for grain yield, while the SCA effects showed that seven hybrids were the best F1 cross combinations such as (CML.317×CML.371) for grain yield.