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In order to investigate the competitive effects of different densities of wild mustard with winter wheat under different levels of nitrogen fertilizer application, an experiment was conducted in the research farm of the college of agriculture - Shiraz University at Kushkak (1998-1999). The experiment was a factorial one with randomized complete block design of 4 replications using additive method. The design consisted of two factors, 5 levels of wild mustard densities (0, 10, 20, 30, 40 plants m-2) and 5 levels of nitrogen fertilizer (0, 50, 100, 150, 200 KgN ha-1), provided from urea. Wheat density was constant (140 Kg ha-1), the cultivar being Marvdasht. Results indicated that in all densities of wild mustard, by increasing nitrogen fertilizer application until 100 Kg N
ha-1, height, tillers per plant, spike length, spike lets per spike, grains per spike, 1000 seed weight, biological yield and grain yield of wheat significantly increased, the increase in grain yield being even more than wheat biological yield. However, at 150 and especially at 200 Kg N ha-1, in plots containing wild mustard, these components decreased or did not change significantly. This indicated that wild mustard competition decreases exploitation of additional nitrogen fertilizer application in wheat. On the other hand, in all levels of the fertilizer, by increasing wild mustard density from 20 plants
m-2, these components significantly decreased and the decrease in grain yield was even more than wheat biological yield. In fact, in higher densities of the weed, increasing nitrogen fertilizer application didn’t decrease the negative effects of weed infestation on wheat yield. By comparing the variation trend of physiological indices, it was noticed that increasing wild mustard densities and decreasing nitrogen levels caused significant decrease in these indices. However, the differences were mostly observed at the peaks of the curves.