In this study, patients histologically confirmed stage II, IIIA or IIIB and who received D2 resection were randomly assigned to receive S-1 for 6 months or S-1 for 1 year. Patients aged from 18 to 75 years and adequate organ function, are randomized 1:1 to S-1 for 6 months and S-1 for 1 year. Both are the 3-week recycle of S-1 (80-120mg per day) for 2 weeks ,followed by 1 week of rest. The primary end point is 3-year DFS, and secondary end point is 5-year OS and safety. Final study analysis will be conducted in the end of the 5th year after the last patient's enrollment.In summary, we hold the hypothesis that S-1 for 6 months is equally effective, safer, and easier to carry out. If possible, there will be a new adjuvant chemotherapy strategy for gastric cancer patients after D2 resection.