The study aims to compare the efficacy and safety of S-1 for 6 months versus S-1 for 1 year as adjuvant chemotherapy after D2 resection in patients with gastric cancer.

Hypothesis: For gastric patients after D2 resection, S-1 for 6 months shows non-inferiority to S-1 for 1 year in disease free survival(DFS), overall survival (OS) and safety.

It has been identified that S-1 is an effective adjuvant treatment for East Asian patients who have undergone a D2 dissection for locally advanced gastric cancer（GC) in the Japanese Adjuvant Chemotherapy Trial of TS-1（S-1) for Gastric Cancer (ACTS-GC) trail , And S-1 has become one of the standard therapies to these patients. But it is still unknown whether it would improve equally or even more to overall survival(OS) and disease free survival(DFS) than S-1 for 1 year compared with S-1 for 6 months. As a result , An further clinical trail is still needed, This trial is designed to investigate the efficacy and safety of S-1 for 6 months versus S-1 for 1 year as adjuvant chemotherapy after D2 resection in patients with gastric cancer.

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.

To ensure the quality of the study, two interim analyses will be planned at the half and the completion of the study respectively. The DATA and Safety Monitoring Committee will independently review the interim analysis and stop the study ahead of schedule if necessary. Furthermore, to improve the study progress and quality, the in-house interim monitoring will be performed.

Inclusion Criteria:

1. Lower age limit of research subjects 18 years old and upper age limit of 75 years old.
2. Be proven to be primary adenocarcinoma of gastric cancer and staged II，IIIA or IIIB by pathological evidences
3. R0 surgery with lymphadenectomy
4. Without any other malignancies
5. ECOG (ECOG score standard) performance status of 0 or 1 and expected to survive more than 6 months
6. No contraindications to chemotherapy, including normal peripheral blood routine, liver and kidney function and electrocardiogram （WBC≥4.0 x 109 /L, NEU≥1.5 x 109 /L,PLT≥100 x 109 /L and HGB≥90g/L).

Exclusion Criteria:

1. Female in pregnancy or lactation, or refuse to receive Contraception measures during chemotherapy.
2. Patients with stage I, IIIC and IV.
3. Unavailable for R0 resection and D2 lymph node dissection.
4. Suffering from other uncontrolled diseases, such as other tumors, acute and Chronic infection.
5. With severe heart disease, including: congestive heart failure, uncontrolled arrhythmias, unstable angina, myocardial infarction, severe heart valve disease and resistant hypertension.
6. Any Known or suspected history of drug allergy test.
7. The researchers believe the patient is not able to complete the entire course of the experiment.
8. Patients (within 4 weeks) are receiving any other anti-cancer drugs therapy, biological therapy, radiation therapy, or Immunosuppressive therapy.
9. Patients conform to any of the following: post-organ transplant, necessary for long-term immunosuppressive or suffering with autoimmune diseases.