We investigated change in platelet activation using flow cytometry in patients before and after splenectomy with total gastrectomy for gastric cancer. Methods: Six patients who underwent splenectomy for lymphadenectomy with total gastrectomy for gastric cancer were the subjects in this study. In the patients, platelet count and platelet activation were evaluated before the operation, 1 week after the operation, and 1 month after the operation. Expression of CD62P (P-selectin) was analysed as a marker of platelet activation using flow cytometry. Results: Although platelet count significantly increased 1 week after the operation, the platelet count 1 month after the operation did not increase significantly. Expression of CD62P (P-selectin) significantly decreased at 1 week and 1 month after the operation, compared with the level before the operation. No postoperative complications occurred in any patient. Conclusion: In the present study, platelet activation did not progress after the operation. The results mean that the risk of thrombosis after splenectomy does not increase.