PURPOSE: The purposes of this study were to develop a computer-assisted surgery system for percutaneous screw fixation of the sacro-iliac joint and to evaluate its accuracy. MATERIALS AND METHODS: We have developed a navigation system composed of an optical tracking device (Polaris, Northern Digital, Canada) and a personal computer. The registration error and target localization error at hybrid registration were measured using a phantom. The errors were measured 30 times for each. Sixteen 6.5 mm cannulated screws were inserted into four plastic bone models (Sawbones, USA), and the accuracy was evaluated. RESULTS: The registration error was 0.76 ±0.33 mm, and the target localization error was 1.43 ±0.42 mm. All of the 16 screws were inserted well across the sacro-iliac joint, and there was neither penetration of the cortical bones nor collision between screws or washers. CONCLUSION: The accuracy of the developed system was similar to existing ones, and its usefulness and helpfulness was proven with screw insertion into plastic bone models.