The purpose of the study is to evaluate the efficacy and safety of postoperative adjuvant chemotherapy with PD-1 inhibitors and chemoradiotherapy, in comparison with adjuvant chemotherapy only, in D2/R0 resected pN3 gastric or gastroesophageal junction adenocarcinoma. PD-1+CRT cohort: A total of 216 patients will receive 6 weeks of PD-1 inhibitors and chemotherapy, then receive concurrent chemoradiotherapy, followed by 6 weeks of PD-1 inhibitors and chemotherapy, finally receive maintenance treatment of PD-1 inhibitors until (maximum 1year after radiotherapy). CT cohort: A total of 217 patients will receive 6 months of chemotherapy. The disease-free survival(DFS), overall survival(OS) and adverse effects will be analyzed.

Inclusion Criteria:

• Eastern Cooperative Oncology Group (ECOG) Performance Status (PS) score of 0 or 1
• Patients with expected survival time more than 6 months
• Patients after standard D2/R0 resection
• Postoperative histologically confirmed adenocarcinoma of the stomach or GEJ
• Positive lymph nodes more than 7, stage pN3
• Patients without distant metastasis (M0) or M1 with abdominal exfoliated cell detection positive (CY1P0)
• Patients' physical condition and visceral function allows following adjuvant therapy, including chemotherapy, chemoradiotherapy and PD-1 inhibitor therapy.
• Patients' blood routine and biochemical indicators should meet the following standard: Hb≥90g/L, ANC≥1.5*10^9/L, PLT≥100*10^9/L, ALT & AST≤2.5 U/L, TB ≤ 1.5 UNL, serum creatinine<1 UNL.
• Patients who are willing to obey regimens during the study.
• Written informed consent is acquired before random entry, and patients should know that he/she has the right to quit, and following treatment won't be affected.
• Patients are willing to provide samples of blood and tissue.

Exclusion Criteria:

• Patients with gross peritoneal metastasis (CY1P0 excluded) or distant metastasis.
• Patients who has received any anti-tumor therapy before surgery.
• Patients who had received radiotherapy for abdominal organs including stomach, liver, kidney, etc.
• Patients who had active systematic autoimmune diseases which need systematic treatment within 2 years before first medication in the study, substitutive therapy (such as thyroxine, insulin, etc) excluded.
• Patients diagnosed with immunodeficiency, or was receiving systematic glucocorticoid treatment or other immunosuppressive therapy within 7 days before medication, physiological dose of glucocorticoid is allowed (≤10 mg/d prednison or equivalent medication)
• Patients who have known severe allergic reaction (≥level 3) to anti-PD-1 monoclonal antibody, 5-FU, Oxaliplatin or any auxiliary material.
• Patient diagnosed with other malignant tumor in the past 5 years, excluding radical basal cell carcinoma of the skin and/or radical resected carcinoma in situ.
• Patient with severe vital organ failure.
• Pregnant or lactation period
• Patient with known mental illness or drug abuse that may influence compliance.
• Patient with known HIV infection, or active tuberculosis.
• Untreated active hepatitis B
• Patient with active HCV infection
• Uncontrolled complications
• Other situations that might disturb study results and compliance.