Drought stress during reproductive stage can have a severe impact on soybean yield. This study will attempt to clarify the effect of droughts on sink-source balance and on the allocation of soluble sugar, starch, and protein to seeds. Soybean (cv. Taegwang, a determinate type) plants were placed under drought stress for 15 days after pod setting, provided with one-day irrigation, and drainage at every four or five days. The seed and the pod weight of a whole plant was severely reduced by 38.7% and 37.8% under drought, respectively, however 100-seed weight and 100-pod weight was relatively slightly suppressed by 9.1% and 11.3% in a drought-stressed plant, respectively. The reasons for these responses of soybean to drought were because pod and seed number decreased in a drought stressed and ratio of leaf area to pod and to seed weight of a drought stressed was clearly similar to that of a well-watered at all time. There was no significant difference in total non-structural carbohydrates and protein content of seeds between drought-stressed and well-watered soybean plants. These results imply that soybean plant regulates balance between sink and source under drought stress and yield reduction by drought during the full-pod stage is mainly due to seed and pod number decreases.