In order to investigate the possible relationship between endogenous hormones and seed filling in soybeans, concentrations of abscisic acid (ABA), gibberellins (GA3), indole-3-acetic acid (IAA), and cytokinins (ZR) in seed, leaf, and pod wall were determined during seed filling of 3 soybean cultivars differing in seed size and quality. All cultivars were grown at 3 densities. The large-seeded cultivar had a strong and greater ability to accumulate photosynthate during seed filling. The genetic trait of seed size was fully expressed at low density. The large-seeded cultivar had a much higher ABA concentration in seed than the moderate and small-seeded cultivars before physiological maturity. ABA concentration in the large-seeded cultivar seed was 40% greater than that of the small-seeded cultivar at 30 days after flowering. Higher densities increased ABA concentrations in seeds. Two peaks of seed GA3 concentration were observed during seed filling. GA3 concentrations at all densities were similar. The peaks of IAA concentration in the 3 cultivars uniformly occurred at 50 days after flowering. The large-seeded cultivar had greater peak concentrations of GA3 and IAA in seed than the other cultivars, while the peak concentration of ZR was highest in the small-seeded cultivar. The concentrations of ABA in leaf increased with time while that of GA3 decreased. The large-seeded cultivar had higher ABA and IAA concentration in leaf while the small-seeded cultivar consistently had higher GA3 concentration in leaf. ZR was present in a smaller amount in the leaf, and was not detected in the pod wall. The large-seeded cultivar maintained higher IAA concentration in pod wall. ABA concentration in seed was positively correlated with seed-filling rate (P < 0.01, r = 0.85**, 0.92**, and 0.83** for large-, moderate- and small-seeded cultivars respectively).The concentration of GA3 in seed was significantly correlated with the seed-filling rate in large- and moderate-seeded cultivars (P < 0.01, r = 0.87**; P < 0.05, r = 0.63*), and no correlation was found for the small-seeded cultivar. There was no correlation between the concentrations of seed IAA, ZR, and seed-filling rate. There was a parallel relationship between seed growth and leaf/pod wall ABA concentration. Thus, ABA might offer a driving force for photosynthate phloem unloading in the seed coat. Lower concentration of ABA and GA3 in the leaf than in seed suggests that most of the two hormones is transported to seed. The mechanism of IAA in seed growth and GA3 concentration and its dynamic in seed quality need further investigation.