A field study was conducted to evaluate the effects of shatter cane (Sorghum bicolor) and soybean (Glycine max) densities on soybean seed yield and to quantify interspecific and intraspecific interference coefficients. The relations between soybean seed yield per plant and shatter cane density at different densities of soybean and also with soybean density at different densities of shatter cane are well described by the reciprocal equations. Soybean seed yield per unit area decreased with increasing shatter cane density. The highest yield loss (57%) was relative to 50 and 12 plants/m2 of soybean and shatter cane densities, respectively. Optimum soybean densities on the basis of maximum soybean seed yield per unit area at 0,4, 8, and 12 plants/m2 of shatter cane were achieved at 50, 34, 32, and 36 plants/m2, as estimated by the asymptotic or parabolic relations between soybean seed yield per unit area and soybean densities at different shatter cane densities. Shatter cane was a stronger competitor than soybean, as a shatter cane plant was equal to 2.5 soybean plants, based on soybean seed yield. In contrast, a soybean plant was equal to 0.18 of a shatter cane plant, on the basis of shatter cane biomass. It was, therefore, concluded that the superior competitor was mostly affected by intraspecific interference, but the weaker competitor was mostly affected by interspecific interference.