Abstract: Objective: To study the role of Id-1 protein in the differentiation of gastric carcinoma.Methods:Tissue chip and immunohistochemical staining were used to detect Id- 1 and Ki-67expression in tumor tissues. MGC803 cells were ex -posed to differentiation-inducing agents (sodium butyrate, NaB). The morphological changes and PAS staining were adopt-ed to confirm differentiation. Id- 1 expression of MGC 803 was assayed by immunocytochemistry and Western blot. Results: The total positive rate of Id- 1 in 89specimens of gastric cancer tissues was 88.8% , and the positive rates were77.4% ,97% and 92% in highly, medium and poorly differentiated carcinoma, respectively, with a significant difference. The positive rate of Ki- 67antigen was 78.7% in all samples. The positive rates of Ki-67antigen in highly, medium, and poorly differentiat-ed carcinoma were 64.5%,90.9% and 80%, respectively, with a statistical significance. Id-1 protein expression was positive -ly correlated with Ki-67antigen expression. Exposed to NaB, the MGC803 cells were obviously improved on the degree of cellular differentiation. The cells became larger, the nuclear cytoplasm ratio was decreased and immunocytochemistry showed less PAS staining. The expression of Id- 1 was sharply decreased as the cells became more differentiated. Conclusion: The expression of Id- 1 in gastric cancer is negatively correlated with differentiation. Id- 1 protein can be an index for grading of gastric cancer.