In order to study the effects of various plant densities and nitrogen fertilizer rates on grain yield, yield components and some morphological traits of two medium-maturity corn (Zea mays L.) hybrids (KSC 604 & KSC 601), an experiment was conducted in 1995 at the experimental field of the Faculty of Agriculture, University of Tehran, Karaj using a split-split plot design with three replications in which, hybrids were in main plots, plant densities as sub plots and nitrogen levels as sub-sub plots. The grain yield per plant as well as well as unit area, grain yield components: kernels per ear, one thousand kernel weight and some morphological traits of plant, ear and cob such as number of leaves per plant, number of leaves above ear, plant and ear height, stem diameter, ear lenght and diameter, number of kernel rows/ear, number of kernels per row, kernel lenght, cob diameter and cob dry matter per plant were measured. The results showed that plant densities and nitrogen levels significantly affected the studied traits, excepting number of plants, above ear leaves and kernel length. The interactions of studied factors were significant for some traits too. Maximum grain yield per hectare was achieved by KSC 601 at 8 plants /m2 with 230 kg N ha-l. This hybrid with the density of 6 plants m-2 and 230 kg N ha-l produced maximum one thousand kernel weight. Plant and ear height as well as stem diameter of KSC 604 were higher than those in KSC 601. Traits such as stem diameter, ear lenght, ear and cob diameter, number of rows/ear, number of kernels per row and ear and one thousand kernel weight were significantly correlated with the grain yield per plant.
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