Purpose: This study was performed to evaluate clinical and radiographical results of treatment of chronic lateral ankle instability with modified Brostrom procedure. Materials and Methods: This is a retrospective study of 22 cases in 22 patients treated with modified $Brostr{ddot{o}}m$ procedure under the diagnosis of chronic lateral ankle instability from May 2000 to August 2004. Average age was 32.3 years. Average follow-up period was 23 months. Preoperative and postoperative radiographs of ankle anteroposterior view, lateral view and varus stress view were analyzed. The clinical evaluation was performed according to the Americal Orthopaedic Foot and Ankle Society scales. Results: After the modified $Brostr{ddot{o}}m$ procedure, Americal Orthopaedic Foot and Ankle Society scales score was improved by average 28 points. Among the 22 cases, 13 cases were excellent, 3 cases good, 4 cases fair, and 2 cases poor with satisfaction of 73%. Five cases had intermittent ankle pain of whom 3 cases had difficulty during competitive sports activity. Lateral tilting of talus improved by average 5.4 degrees on varus stress view. Conclusions: The modified $Brostr{ddot{o}}m$ procedure is one of the most effective methods for treating of chronic lateral ankle instability.