Abstract: This paper first presents the homography between a plane in the scene and a plane of the image, and the absolute conic in space under an affine coordinates. With the homography under an affine coordinates, and using the constraints for circular points to camera intrinsic parameters, we obtain a group of equations from several images containing a planar template, which consists of an equilateral triangle with the inscribed circle. By linearly solving the above equations, we determine the intrinsic parameters. Experimental result shows that our approach is highly accurate.