Using the genetic diversity of crop cultivars to improve crop resistance against various biotic and abiotic stresses is effective way to develop sustainable agriculture. Therefore, aimed to investigate the effect of cultivar type as well as introduce the most important traits related to competitive ability of sunflower cultivars against tumble pigweed, a factorial experiment with three replications was at the agricultural research station, University of Birjand during 2012. Experimental treatment included six cultivars of oily sunflower (Azargol, Syrna, Farukh, Jame Isfahan, Progress and Euroflor) and two different densities of tumble pigweed (zero and 15 plant/m²). ANOVA results showed that there was a remarkable genetical diversity among studied cultivars and significant reductions in studied traits values observed where tumble pigweed was present. Moreover, there were significant positive correlations between traits such as leaf area index, plant height, the rate of leaf area and height development, relative growth rate, seed yield, etc. with the tolerance index. Factor analysis also identified 5 factor groups which justified 79% of the variations. Overall, Jame Isfahan showed the greatest competitive ability against tumble pigweed compared with other studied cultivars.