Evaluation the plant density changes and weeding effects on yield and yield components of maize (KSC 704) in the research farm of Razi University, Kermanshah, Iran in the middle of the growing season were studied. The experimental design was split plot based on four replicated randomized complete block design. Main plot treatments were one and two steps weeding and sub plot levels were ten change plant densities included 88000 plants per hectare (pha-1), 106000 pha-1, change plant density from 106000 to 88000 pha-1 in the 8 and 14 leaf stage, 133000 pha-1, change plant density from 133000 to 106000 pha-1 in the 8 and 14 leaf stage, 177000 pha-1 and change plant density from 177000 to 133000 pha-1 in the 8 and 14 leaf stage. The results of ANOVA indicated the significant effects of plant density changes for ear length, cover weight of the ear, ear weight without cover, seed weight per ear, weight of ear wood, number of seeds per row (p≤ 0.01). The most seed yield per hectare was 16.7 tons that obtained from two steps weeding and change plant density from 106000 to 88000 pha-1in the 8 leaf stage. These results about seed yield indicated that cultivation of maize with plant density higher than plant density used by farmers increase the power of competition against weeds during sowing to 8 leaf stage and decrease plant density to conventional levels in this stage decrease the inter-specific competition.

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