Abstract: The estimation of camera distortion parameters is an important step in the whole camera calibration process, which will directly affect the accuracy of subsequent visual measurement and 3D reconstruction. Currently the distortion estimation in most of the existing calibration methods in the literature is done in a two-step scheme: Firstly the pinhole model is assumed and calibrated, then the distortion parameters are determined. In this work, a homography based method is proposed which could estimate the distortion parameters without resorting to a prior pinhole model estimation step. Our proposed method at first compute the homography from the space plane to the image plane using image feature points around the camera's principal point, by which image distortion errors and distortion parameters are then computed, and a nonlinear optimization step is finally employed to simultaneously estimate both the homography and the distortion parameters. The effectiveness of our proposed method is demonstrated by both simulation and real image experiment. Since the proposed method needs only a single image for estimating the distortion parameters, it can significantly enhance the flexibility of camera calibration.