In this paper, an improved optical flow method for image registration is proposed. It is novel in the way that it improves the optical flow method with an initial motion estimator: extended phase correlation technique (EPCT), using merits of the latter to compensate deficiencies of the former. In a more detailed manner, it can be said that the optical flow method can reach the sub-pixel accuracy and calculate complex distortion patterns like chirping and tilting but is weak with large-scale movements. Because EPCT covers measurements of large translations and rotations with pixel level accuracy and is efficient in the calculating load, it can be treated as a good initial motion estimator for optical flow method. Tests have proved that this improved method will significantly enhance the registration performance, especially, for images with large-scale movements and robust against random noises.