C-arm is a medical imaging modality that provides intra-operative fluoroscopic image. C-arm is frequently used in various medical operations. Recently, there have been many attempts to use C-arm in computer-aided surgery. It is an essential step to calibrate the distortion of C-arm image to use C-arm in this application. The distortion of C-arm consists of pincushion distortion and S？curve distortion. The nonlinearity and dependency on the orientation of C-arm make the distortion calibration procedure laborious and time consuming. In this paper we propose a new distortion calibration method, which requires less effort and time. This method utilizes a distortion model based on physical phenomenon of distortion.