ABSTRACT. This study aimed to 1) analyze the results of tacrolimus blood concentration monitored in patients after renal transplantation, 2) observe and establish an optimal therapeutic window for patients, and 3) provide evidence for the clinical and rational use of drugs. Tacrolimus blood concentration was determined by enzyme-linked immunosorbent assay. A total of 1824 cases were obtained from the monitoring of 74 patients after renal transplantation. These cases were then retrospectively analyzed. Over time, the mean whole blood tacrolimus trough concentration after transplantation gradually decreased. This result suggests that the optimal therapeutic windows for patients with renal transplants are as follows: 5 to 20 µg/L at 1 month after surgery; 5 to 15 µg/L at 1-3 months after surgery; 4 to 12 µg/L at 3-6 months after surgery; 4 to 10 µg/L at 6-12 months after surgery; and 3 to 8 µg/L at >12 months after surgery. The absorption of tacrolimus is highly variable. Therefore, tacrolimus concentration in the blood and the recommended clinical therapeutic window should be routinely monitored to adjust the treatment regimen and reduce adverse reactions. In this way, treatment can be optimized.