Environment registration is a key issue for an augmented reality (AR) system. An efficient markerless registration algorithm is introduced in this paper. The correspondences between frames are obtained in the process of tracking planar target with a fixed template. The homograph of the target can be computed with the perspective projective transformation. Combined with the intrinsic parameters of the camera

pose estimation of the region can be completed resulting in 3D registration for the system. A markerless AR system based on this algorithm is designed. The system architecture and working flow are illustrated. Experimental results demonstrate the accuracy of the theoretical analysis and the robustness of the system.