Purpose: The purpose of this study is to analyze the clinicopathologic characteristics of gastric cancer patients and to evaluate the survival and prognostic factors and effect of immunochemosurgery for gastric cancer patients. Materials and Methods: The clinicopathologic characteristics were analyzed for 12,277 consecutive patients who underwent operation for gastric cancer from 1970 to 1999. We also evaluated the survival and prognostic factors for 9,262 consecutive patients from 1981 to 1996. The prognostic significance of treatment modality [surgery alone, surgery+chemotherapy, surgery+immunotherapy+chemotherapy (immunochemosurgery)] were evaluated in stage III gastric cancer. Results: The 5-year survival rate (5-YSR) of overall patients was $55.8\%$, and that of patients who received curative resection was $64.8\%$. The 5-YSRs according to TNM stage were $92.9\%$ for Ia, $84.2\%$ for Ib, $69.3\%$ for II, $45.8\%$ for IIIa, $29.6\%$ for IIIb and $9.2\%$ for IV. Regarding adjuvant treatment modality, significant survival difference was observed in stage III patients. The 5-year survival rates were $44.8\%$ for immunochemosurgery group, $36.8\%$ for surgery+chemotherapy group and $27.2\%$ for surgery alone group. Curative resection, depth of invasion and lymph node metastasis were the most significant prognostic factors in gastric cancer. Conclusion: Consequently, early detection and curative resection with radical lymph node dissection, followed by immunochemotherapy especially in patients with stage III gastric cancer should be recommended as a standard treatment principle for patients with gastric cancer.