Weeds are reported as one of the most serious problems in red kidney bean production. To evaluate the competitive ability of red bean cultivars with red root pigweed an experiment was conducted at the Research Farm of the University College of Agriculture and Natural Recourses, University of Tehran in 2006. Different densities of pigweed including 0, 4, 8, 16 and 32 plants/m2 along with three cultivars of red bean (AKHTAR, SAYYAD and Line D81083) were employed in a factorial experiment with complete randomized block design. The effects of pigweed density on leaf area index, crop growth rate, biological and grain yield of dry bean were investigated during the study. Results indicated that at a density of 32 plants/m2 of pigweed, cultivar SAYYAD and Line D81083 had the most vs the least leaf area index and crop growth rate, respectively. Differences between grain and biological yield of cultivars were significant with Line D81083 exhibiting the least grain and biological yield. The density of 32 plants/m2 of pigweed, reduced grain yield of cultivars SAYYAD, AKHTAR and Line D81083 about, 42, 57 and 72%, respectively. Grain yields of red bean cultivars were more seriously affected by pigweed interference, in comparison with their biological yields. Based on the coefficients of yield loss models (a & m), red bean cultivar of SAYYAD had the most competitive ability among the red bean cultivars. The Line D81083 suffered from the most grain and biological yield loss. Also cultivar SAYYAD and Line D81083 had the most vs the least effect on biomass and leaf area index of pigweed respectively, indicating that cultivar SAYYAD has the most competitive ability against pigweed.