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Determination of combining ability and gene effects is one of the most important steps in systems relative to inbred development in maize hybrid breeding program. According to this, 22 early maturity maize lines were crossed to 2 early maturity commercial testers. Forty-four line × tester combinations were evaluated under normal (75000 plants/ha) and high (110000 plants/ha) plant densities in 2001 at Mashad center of agricultural research, Iran. The experimental design was randomized complete block. The results indicated significant differences between hybrids for all traits under study. Significant difference between lines for all traits and between testers for plant height, days to silking, days to physiological maturity, number of seeds per row and number of rows on ear in high plant density and plant height, days to silking, days to physiological maturity, kernel depth, number of seeds per row number of rows on ear and grain yield in normal plant density was estimated. This is indicated the role of additive gene effects on controlling these mentioned traits. Upon significant of line × tester mean square, significant dominance variance was estimated for plant height, days to silking, days to physiological maturity, number of total leaves, kernel depth, number of seeds per row and 300 kernel weight in high plant density and plant height, kernel depth and number of rows on ear in normal plant density. The estimates of revealed that the role of non-additive gene effects were high in magnitude the additive gene effects in controlling of plant height, days to silking, days to physiological maturity, number of seeds per row in high plant density and plant height, kernel depth, number of seeds per row, number of rows on ear in normal plant density, and as regards the magnitude of this ratios, the additive component was more important for other characters in each plant density. This indicated the changes of gene expression under each condition. There were differences among lines and testers for general combining ability (GCA) and among crosses for specific combining ability (SCA). For line screening for following of inbreeding, lines L6, L9, L20, L42 and L45 were selected that had high grain yield in both normal or high plant densities, and also had desirable GCA for important traits.