In order to investigate the effects of plant density, application time and dose of herbicide on weed control and soybean yield, a research was conducted in factorial based on randomized complete blocks design with 11 treatments in three replications at Ghorogh village, Gorgan in 2011. The treatments were soybean cultivation of the DPX (Katool) cultivar with two densities of 20 and 40 plants/m² accompanied with application of the herbicide Imazethapyr (500, 750 and 1000 ml/ha) that was used as pre- and post-emergence and Metribuzin with a dose of 600 g/ha that was applied pre-emergence, weeding and without weeding (control). Results indicated that changes of dose and application time of Imazethapyr have effect on density and dry weight of weeds, so that with increase of dose increased the efficiency of Imazethapyr. In both densities at 25 days after Imazethapyr spraying, as post-emergence treatment, at all doses, caused a decrease in dry weight of weeds compared to the treatment applied pre-emergence. The efficiency of Metribuzin at 25 and 40 days after spraying was similar to those of Imazethapyr that was applied with a dose of 1000 ml/ha. The trend of soybean emergence at two densities indicated that weed removal had a greater effects on soybean growth at a density of 20 plants/m² compared to the density of 40 plants/m². Change in soybean density had a significant effect on the yield through plant number, the number of pod in the plant and seed weight. At the density of 20 plants/m², pre-emergence application of Imazethapyr enhanced. The number of soybean pods is slightly more than to its post-emergence treatment. At the density of 40 plants/m², there was no difference between pre-emergence and post-emergence of Imazethapyr.