Outlier mining is a hot topic of data mining. After studying the commonly used outlier mining methods, this paper presents an outlier mining algorithm OMABD(Outlier Mining Algorithm Base on Dissimilarity) based on dissimilarity. The algorithm first constructs dissimilarity matrix based on object dissimilarity of each object of data set, then makes the dissimilarity degree of each object according to the dissimilarity matrix, and finally outlier will be detected by comparing the dissimilarity degree with dissimilarity threshold. The experiment results show that this algorithm can detect outlier efficiently.