Abstract: To solve the mismatch problem caused by big initial registration error in image registration algorithm based on implicit similarity, the algorithm of hierarchical combining registration algorithms based on explicit similarity and implicit similarity for multimodal image is proposed in this paper. Pyramid transform is first adopted to obtain low resolution images to be registrated. Mutual information is used as explicit similarity, and Particle Swarm Optimization (PSO) algorithm is adopted to compute the initial registration parameters. The algorithm based on implicit similarity is then used to obtain more accurate results by combination of PSO and Powell algorithm. The registration parameter above is used to transform the high resolution image to be registrated first, and registration parameter is then optimized using the method above, where the final registration parameter is computed using these two registration parameters. Several sets of visible images, visible and infrared images, and multiband SAR images are used to perform registration experiments. The experimental results demonstrate that, the proposed algorithm can be applied to multimodal image registration, reduce registration errors, and have certain applicability.