Two field experiments were carried out at the Agricultural Research and Experiment Station, Fac, of Agric. at Moshtohor in 1979 and 1980 seasons. This experiment included 8 treatments which were weedy for 21, 42, 63 84, 105 days from planting beside weedy throughout the growing season, hoeing and weed—f ree throughout the grown season. The tallest plants were obtained by weed-free throughout the growing season and hoeing treatioents followed by weedy for 21 days from sowing. Seed yield was reduced by leaving weeds to compete soybean plants over 21 days from planting. The depression reached to 50% approximately or more by weedy periods at 63 days from planting.