Soybeans (Glycine max L.) are often planted as a double crop later than would be optimal to produce the highest yield. Growers often apply more seeds of cultivars which were developed for a conventional full-season practice than a standard plant density to increase seed yield. High population density of the conventional cultivars during a short season has often resulted in productivity decline. The objective of this study was to determine the impact of plant density on short and long internode soybean genotypes for agronomic and physiological traits in order to develop breeding criteria. Field experiments were conducted over a period of three years using five genotypes which differed in internode length and branch number. All plants from middle two rows were harvested for agronomic traits, including mean seed yield (kg ha-1), sink capacity, and harvest index. Short internode genotypes (Camp and HS 287) significantly (P<0.05) increased in plant height up to 18 % as compared to the long internode ones in high plant density. Number of pods on the peduncle clearly increased up to 93.9 % in long internode genotypes as the number of plants per unit area increased. Total plant dry biomass in one of short internode genotype, Camp, significantly (P<0.05) increased from 7,240 to11,082 kg ha-1 (up to 53.1 %) with increasing plant density. Harvest index (HI) increased in all short internode genotypes up to 0.35 in Camp and to 0.42 in HS 287 in high plant density. Relatively high HI in Camp and HS 287 suggests that short internode genotypes were genetically stable for a heavy competition among plants in a short growth season. The major factors responsible for increased yield in high plant density were an increase in number of pods per unit area and number of pods on peduncle of genotypes particularly with short internode.