In order to evaluate the competitive effects of wild oat on winter wheat in different densities, a field experiment was conducted in Mashad in 2001 .The experiment was carried out as factorial in a randomized complete block design with four replications. The factors included wheat densities at 3 levels (300,450,600 plants/m2) and wild oat densities at 5 levels (0, 20, 40, 80,160 plants/m2). Results indicated that wild oat plant height was lower in the early stages of growth but increased in the latest stages compared to wheat. Wheat plant height increased with increases in wheat plant density. Increase in wheat plant density led to decrease in wild oat LAI and biomass. Time of canopy closure in wild oat reached faster than winter wheat (nearly 15 days).The average wheat and wild oat relative growth rates during the growth period were 0.027 and 0.034 g-l g-1 day-l, respectively. The maximum amount of wheat yield loss was nearly 47% in 160 wild oat and 300 wheat plants/m2 and in the presence of wild oat, wheat yield loss decreased as wheat plant density increased