In this paper, we show how to learn a good similarity metric for SVM classification. We present a novel approach to simultaneously learn a Mahalanobis similarity metric and an SVM classifier. Different from previous approaches, we optimize the Mahalanobis metric directly for minimizing the SVM classification error. Our formulation generalizes the traditional large margin principle used in standard SVM, that is, we maximize the margin-radius-ratio. The learned similarity metric significantly improves the classification performance of standard SVM. Empirical studies on real datasets show the proposed approach achieves higher or comparable classification accuracies compared with state-of-the-art similarity learning methods.