To investigate the effects of lambsquarters density and its relative time of emergence on yield and yield components of grain corn a two year experiment was conducted in 2001 and 2002 growing seasons in the research field of Weed Research Department, Plant Pests and Diseases Research Institute, Karaj. Both experiments were conducted using factorial arrangement of treatments in a randomized complete block design with 3 replications. The treatments were the relative time of lambsquarters emergence (1st year: simultaneously, at 2-3 and 4-5 leaves stages of corn; 2nd year: simultaneously, at 2-3 and 5-6 leaves stages of corn) and lambsquarters density (first year: 2, 5, 10 and 15 plant.m-2; second year: 6.6, 13.3 and 20 plant.m-2). A pure plot of corn was also cultivated to evaluate grain yield of corn in weed free conditions. The results indicated that both treatments had significant effects on grain and biological yields, harvest index, grain number/ear row and row number/ear of corn. Thousand-seed weight was not affected by lambsquarters density. The results also showed that the negative effect of interference period on corn was more than that of density. In conclusion, a delay in lambsquarters emergence and a good management against it at the beginning of the growth season can increase the competitive ability of corn against lambsquarters.