Abstract: To investigate the distribution and expression of protein P27 and phosphorylated P27 ( p-P27 ) in gastric cancer cells BGC823, when P27 becomes over-expressed. Methods: The whole length of p27KIP1 cDNA was transfected into human gastric cancer cells BGC823 via lipofectin transfection. Cell cycle and apoptosis were determined by flow cytometry, and distribution and expression of protein P27 and p-P27 were analyzed by Western blot and immunofluorescence. Results: Protein P27 over-expression caused cell arrest and increase in the G1 population ( transfected cells vs. control cells was 62.783% vs. 41.933%, P = 0.000 ). It also induced apoptotic cell death and increased apoptotic index (transfected cells vs. control cells was 26.3 vs. 3.6, P = 0.000 ). The expression of P27 and p-P27 evidently increased in transfected cells. In the control cells, P27 was expressed in the nuclei, and p-P27 in the cytoplasm. On the other hand, P27 was expressed in the nuclei; and p-P27 in the nuclei and cytoplasm ( but mainly in the nuclei ) in transfected cells. Conclusion: Over-expression of P27 induced the arrest of gastric cancer cells in G1 and caused cell apoptosis. The levels of P27 in nuclei and the ratio of p-P27 in nuclei and cytoplasm were also assumed to be important factors in evaluating tumor proliferation.