The purpose of this study was to investigate the efficacy of cilostazol for prevention of thrombosis in microsurgical anastomosis. We used not only a patency test, but also ultrasonography to evaluate the blood flow volume (FV) more accurately and objectivity. In addition, we evaluated the accuracy and sensitivity of the patency test. Thrombogenic anastomoses were performed on rat external iliac arteries. Forty-six rats were randomly allocated into the following 5 experimental groups: negative control, without any surgery and medication; control, received distilled water orally; group A, received aspirin orally; group B, received cilostazol orally; and group C, received aspirin and cilostazol orally 3 hours before a thrombogenic artery anastomosis. The artery was evaluated by FV using ultrasonography and a patency test 10, 30, and 120 minutes after clamp removal. The FV in group C was significantly larger than the control group 30 minutes after clamp removal. The FVs in groups A, B, and C were significantly larger than the control group 120 minutes after clamp removal. The FV decreased in all groups with time. The patency rate between the control group and group C 120 minutes after clamp removal was significantly different. There were 8 false negatives in a total of 114 patency tests. The total accuracy and sensitivity of the patency test were 93.0% and 92.7%, respectively. Cilostazol plus aspirin therapy is recommended for reducing thrombosis and increasing FV without increasing the risk of bleeding during microsurgery. The patency test cannot evaluate the state of the anastomotic vessels accurately and objectively.