Determination of gene effects and combining abilities is a critical stage in maize hybrid breeding, and for this purpose inbred lines of maize G17, Inbreed12, ZM51, Th97Allah-K122, G105 and G54 were used as male parents in this study and crossed with the testers ZM47R, CA21R and ZM49R as female parents. Parents and the resulting hybrids were evaluated in a randomized complete block design with three replications at three locations (Sharqat, Hawija and Mosul) through autumn season of 2013. During the growing and maturity periods, data were collected for dates of tasseling and silking, plant height, ear length and diameter, number of rows and grains per ear, 100 grain weight, grain yield per plant, oil percent and protein percent. The results (as average of the three locations) showed that mean square of lines and testers were highly significant on all the characters except oil percent for lines indicating the importance of additive gene effect in controlling its inheritance. The two inbred lines Th97Allah.K122 and G105 were identified as good general combiners for grain yield per plant because they showed significant desired general combining ability effects for this character. Th97Allah.K122 was superior compared with G105 because of significant positive GCA for most other characters, then two lines have potential additive gene effects to be utilized in the breeding programs. CA21R tester showed favorable additive gene effects for grain yield per plant, some of its components, early maturity and oil percent. The highest grain yield per plant (195.22 gm) with significant desirable specific combining ability effect belonged to ZM47R × Th97Alla-K122 combination, in addition to its significant desirable specific combining ability effects for dates of tasseling and silking, ear diameter, number of rows and grains per ear and 100 grain weight, followed by crosses (CA21R x Inbreed12), (CA21R x G54), (ZM49R x G17) and (ZM49R x G105). Additive genetic variance was substantially higher than dominance genetic variance for all characters except ear diameter and oil percent, indicating that additive gene effects were more prominent than dominance effects in controlling its inheritance, therefore, narrow sense heritability estimates closely resembled the broad sense heritability values especially for dates of tasseling silking and plant height. Narrow sense heritability values ranged between 0.1229 for oil percent and 0.6827 for 100 grain weight, and was high for plant height, number of grains per ear and 100 grain yield, medium for dates of tasseling silking, ear length, number of rows per ear and grain yield per plant and low for other characters, and medium to high narrow sense heritability estimates enable to select for favorable additive gene effects among the studied lines.