Camera calibration is a fundamental task in photogrammetry and computer vision. This paper presents an approach for geometric calibration of digital cameras. The mapping relation between the image coordinate and the world coordinate can be described by a polynomial. The coefficients and ranks of the polynomial are computed by the minimal 6 corresponding points in the two coordinate systems. This method can calibrate the camera directly without calculating the camera's intrinsic parameters and extrinsic parameters, which decreases the complexity of calculation. Experimental results show the precision and robustness of the presented method.