To determine the effects of pigweed density on yield and yield components and morphological characteristics of soybean cultivars, a factorial experiment in randomized complete block design form was carried out with three replications. The treatments included soybean cultivars (L11, Clark and Pershing) and pigweed densities (controlled plots, 4, 8, 16 & 32 pigweeds in a meter of row) in soybean plots and plots of pure pigweed. Results showed that between soybean cultivars were different in measured characteristics. Increasing pigweed density decreased grain yield, biological yield, harvest index, number of pods and branches in soybeans. But seed per pod, 100-seed weight, plant height, number of nod in main stem were not affected by pigweed densities. The most reduced grain and biological yields was observed in Pershing and Clark cultivars, respectively. Increasing pigweed density had also declined pigweed plant biomass. Pigweed biomass was affected by competitor soybean cultivars. In Pershing cultivar, this cractristic was less than other cultivars, significantly.