Field experiment was conducted to evaluate the effect of competition of different wild oat densities on wheat cultivars using replacement and additive series designs. Factorial experiment with randomized complete block design was used with 3 replications in research station of college of agriculture, Shiraz University in 1997. The design consisted of wild oat densities (0, 63, 125, 188, 250 plants m-2) and three wheat cultivars with constant density of 250 plants m-2 (CROSS AZADI, NIKNEJAD and DARAB2 with heights of 100-1\5, 95-100 and 75-85 cm, respectively). With increase in density of wild oat, final height of wheat cultivars was reduced, but it was not statistically significant. In all wild oat densities, the difference in height of wild oat and DARAB2 was significant. DARAB2 showed the greatest decrease in tiller number. Number of wild oat tillers in competition with this cultivar was significantly more than other cultivars. Yield and it*s components and also harvest index (HI) were affected by wild oat densities and decreased significantly. At higher densities, number of spikelets in panicle and 1000 seed weight of wild oat in mixture with CROSS AZADI were minimum while these were maximum in mixture with DARAB2. At the highest wild oat density, grain yield in CROSS AZADI, NIKNEJAD and DARAB2 was reduced 66, 73 and 88 percent, respectively.