This paper presents accurate algorithm for geometric calibration of X-ray imaging system. Calibration is a very important process for improving system performance. There has been a lot of previous works using linear camera modeling tachnique, where lens distortion is neglrected and/or center of distortion should be calculated. This paper presents a new calibration method to estimate the intensifier position and orientation, scale factor, distortion coefficient, magnification factor, and center of distortion using the least square method. we investigate the properties of the algorithm by computer simulation. Simulation results show that the parameters can be estimated accurately using the proposed algorithm.