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Effects of nitrogen starter fertilizer and plant density on yield and oil and protein content of soybean (Glycine max L. Merr) are not well understood, because nitrogen starter fertilizer and plant density has been tested separately. Two years field experiment was conducted to evaluate effects of these factors on yield, yield components, oil and protein content in 2006 and 2007 in Kermanshah, Iran. The experiment was conducted on soybean (var. Williams) as a split-plot based on randomized complete blocks design with three replications. Nitrogen starter fertilizer treatments were arranged in three rates (0, 40, 80 kg ha(-1)) as main plots and plant density as sub plots arranged with three levels (15, 30, 45 plant m(-2)). Based on similarity treatments and experimental designs, the results of analysis of combined variance and mean comparisons showed significant (528.4 kg ha(-1) yield increase as density increased from 30 to 45 plant m(-2) and nitrogen starter fertilizer increased from 0 to 40 kg ha(-1) in two years. Analysis of correlation showed a positive significant correlation between yield and number of seed per plant (r = 0.724), number of pods and yield (r = 0.463), thousand seed weight and yield (r = 0.437). A linear regression was found between yield and number of seed per plant, number of pods and thousand seed weight (yield = 37.58 + 0.73x1-0.14x2 + 0.7x3; r2 = 0.56); p < 0.01). Seed protein was unaffected by plant densities, but nitrogen application changed it. Dissimilarly, oil content has a diverse respond to treatments. This experiment showed density of 45 plant m(-2) and application of nitrogen starter fertilizer 40 kg ha(-1) are optimum and increase grain yield under condition of our experiment. We suggest to conduct some experiments for understanding of linear relationship for number of pod for understanding of linear relationship for number of pod for levels of nitrogen starter and quadratic relationship for number of seed for levels of density.

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