Plant density in a crop community is an important determinant of yield. However, the yield potential of soybeans was not changed at the so-called optimum plant densities, from 16～22 plants/m². Field experiments of split-plot design, in which 5 soybean cultivars were taken as the main plotand 4 plant densities as sub-plot, were conducted to determine the possible reason of the relative stability of soybean yield. It was shown that the yield of plot community for each of the cultivars wasnot increased significantly with the density increase from 16 to 92 plants/m². This is due to the decrease of the yield/plant, including pods and seeds/plant with the increase of the density but not 100-seed weight and seeds/plant.