Purpose: The purpose of this study is to investigate the prognostic significance of tumor size for 5-year survival rate in patients with gastric cancer. Materials and Methods: A total of 1,697 patients with gastric cancer, who underwent potentially curative gastrectomy, were evaluated. Patients were divided into 4 groups as follows, according to the median size of early and advanced gastric cancer, respectively: small early gastric cancer (tumor size ${leq}3$ cm), large early gastric cancer (tumor size >3 cm), small advanced gastric cancer (tumor size ${leq}$ 6 cm), and large advanced gastric cancer (tumor size >6 cm). The prognostic value of tumor size for 5-year survival rate was investigated. Results: In a univariate analysis, tumor size is a significant prognostic factor in advanced gastric cancer, but not in early gastric cancer. Multivariate analysis showed that tumor size is an independent prognostic factor for 5-year survival rate in advanced gastric cancer (P=0.003, hazard ratio=1.372, 95% confidence interval=1.115~1.690). When advanced gastric cancer is subdivided into 2 groups, according to serosa invasion: Group 1; serosa negative (T2 and T3, 7th AJCC), and Group 2; serosa positive (T4a and T4b, 7th AJCC), tumor size is an independent prognostic factor in Group 1 (P=0.011, hazard ratio=1.810, 95% confidence interval=1.149~2.852) and in Group 2 (P=0.033, hazard ratio=1.288, 95% confidence interval=1.020~1.627), respectively. Conclusions: Tumor size is an independent prognostic factor in advanced gastric cancer irrespective of the serosa invasion, but not in early gastric cancer.