This research proposes a measure called cluster similar index (CSI) to evaluate the similarity of cluster for discrete elements. The CSI is used as a criterion to build the automatic fuzzy clustering algorithm. This algorithm can determine the suitable number of clusters, find the elements in each cluster, give the probability to belong to the clusters of each element, and evaluate the quality of the established clusters at the same time. The proposed algorithm can perform quickly and effectively by the established MATLAB procedure. Several numerical examples illustrate the proposed algorithm and show the advantages in comparing with the existing ones. Finally, applying the proposed algorithm in the image recognition shows potentiality in the reality of this research.