In this paper, a novel, fully automatic camera calibration procedure is proposed to estimate the camera principal point, i.e., the intersection of the optical axis and the image plane. The basic idea is to derive the orthogonal projection of the camera optical axis onto the flat-screen monitor via changing/analyzing simple edge patterns displayed on the flat monitor. The principal point can then be estimated as the intersection of images of line features thus derived on the screen, called calibration lines, from multiple monitor configurations. Thus, the calibration can be performed automatically using fixed camera and multiple monitors, or using multiple poses of a movable monitor. Several experiments are developed to evaluate the proposed method for accuracy as well as robustness, and the results show that the proposed approach compares favorably with some previous works, including the classic Zhang's algorithm that derives the principal point together with other camera parameters at the same time.