Objective To evaluate the treatment of supra-condylar fractures of the humerus (12-B1.2 by AO classification) with a unilateral external fixator. Methods From 2003 to 2006,28 cases of supra-condylar fractures of the humerus were treated with a unilateral external fixator. They were 15 males and 13 females. Their average age was 43 (24 to 58) years old. They were all closed fractures and 2 cases complicated with radial nerve injury. Results All were followed up for an average of 28 (12 to 54) months. All fractures united with an average healing time of 4.5 months. The 2 cases of radial nerve injury recovered completely. Five cases had infection of pin holes. The patients had an average elbow score (Money and Bryan system) of 97 and an average shoulder score (Constant and Murley system) of 98. Conclusion The unilateral external fixator is appropriate for supra-condylar fractures of the humerus ( 12-B1.2 by AO classification),because it provides not only effective fixation but also early functional rehabilitation.