Abstract: A diallel cross among inbred lines of maize (Zea mays L.) with medium maturity and an evaluation to estimate heterosis and genetic parameters for ear length (cm), ear diameter (cm), No. of rows ear-1, No. of kernels row-1, weight of 100-kernels and grain yield (ardab feddan-1) were carried out. Hybrids mean squares were highly significant for the six traits under both planting dates and combined analysis, except ear diameter. Indicating that the hybrids performance are differed from planting date to another. Results indicating that the parental inbred P5 gave high grain yield. The highest grain yield was obtained from crosses (P4xP1) and (P5xP4) in combined, these crosses were significantly out yielded the two checks SC 155. SC 162 at 5%. Moreover, crosses (P1xP4), (P1xP5),(P6xP1), (P4xP2), (P2xP3) and (P3x P6) were insignificantly better than the checks. For ear length, P1xP4 (56.67 and 67.65%) showed maximum positive heterosis over better-parent and mid-parent, respectively. For ear diameter P2xP1 gave maximum positive heterosis over better-parent and over mid-parent, for No. of Kernels row-1 26 crosses had highly significant and positive heterosis over mid-parents, for No. of Kernels row-1 six crosses (P1x P2),(P2x P1), (P4 x P1), (P1 x P5), (P5xP1) and (P2xP6) had highly positively significant heterosis over better-parent. The highest positive significant heterosis over mid-parents for 100-kernel weight were recorded by 14 crosses. Cross P1xP2 showed maximum positive and significant heterosis over better-parent and mid-parents and only one cross P1xP4 gave positive and significant over check varieties for 100-kernel weight. For grain yield, cross P2xP4 (289.55%) and (295.73%) showed maximum positive and significant heterosis followed by P4xP2 (267.22 and 273.04%). Heritability in the narrow sense was the highest in No. of rows ear-1, while heritability in the broad sense was the highest in ear diameter. Heritability estimates in broad sense were medium (33-66%) of all studied traits except for ear diameter was high estimate (71%). Hence it could be concluded that these crosses may be useful for improving maize grain yield program.