A trial was conducted at the farm of agriculture in Abu-ghraib, for two seasons, spring and autumn 2012. The objective was to study the gene action and combining ability of diallel crosses hybridized at two nitrogen rates (N100 and N400 kgN.ha-1). At first season, diallel cross with one direction was done among four maize inbred lines viz: Zm7, Zm607, Oh40 and Zr8 to produce six crosses for each nitrogen rate. Yield trail between crosses and their parents was conducted by RCBD with three replicates at the autumn season of 2012. Means of characters were compared and the results showed that there were significant differences among crosses in most characters studied. The cross P2 x P3N100 gave highest grain yield amount to 14.86 ton.ha-1 ,as a result of its superior in number of kernels per ear and 300 kernels weight. Also, the same cross gave highest hybrid vigor in grain yield, biological yield and harvest index amount to 117.8%, 83% and 13.3%, respectively. Contrast values N400 Vs N100 showed there were significant differences among crosses hybridized under nitrogen rates in number of ears per plant and number of kernels per ear. The P4 was superior in positive effects of gca in grain yield, biological yield, and number of kernels per ear. Therefore P3 was the best combiner in breeding for high yield hybrids. The best cross in specific combining ability (sca)was P2xP3N100 in grain yield, biological yield, 300 kernels weight and number of kernels per ear. Dominance variance was greater than additive variance in most characters studied in coincidence with average degree of dominance more than one. The highest values of heritability in broad sense were recorded in number of kernels per ear, 300 kernels weight, grain yield and biological yield. Heritability in narrow sense was high in number of kernels per ear. There were no significant differences of gca and sca in number of ear per plant and N-utilization efficiency. It will be concluded that hybridization can be performed under low nitrogen input will not decrease hybrid vigor, furthermore, it saves half of nitrogen applied, especially; there was no increasing in yield when these crosses were grown under high nitrogen input.