Gray scale joint image similarity degree is defined according to the morphology method. Two kinds of modified morphological correlation algorithm based on bit representation decomposition are presented. The gray scale image is decomposed into a set of binary slices in the bit representation decomposition. In the first algorithm, the edge of each pair of slices is detected. In the second algorithm, the joint power spectrum of each pair is summed after binarized or the summation of the joint power spectrum of these pairs is binarized. Computer simulation results indicate that the modified algorithms can improve the discrimination capabilities with respect to the high similarity gray scale images.