It has been shown in prior work in management science, statistics and machine learning that using an ensemble of models often results in better performance than using a single ‘best’ model. This paper proposes a novel Data Envelopment Analysis (DEA) based approach to combine models. We prove that for the 2-class classification problems, DEA models identify the same convex hull as the popular ROC analysis used for model combination. We further develop two DEA-based methods to combine k-class classifiers. Experiments demonstrate that the two methods outperform other benchmark methods and suggest that DEA can be a powerful tool for model combination. ______________________________ A preliminary version of this paper was accepted at the ACM Conference on Knowledge Discovery and Data Mining 2004 (KDD04). This version substantially extends the conference publication by a comprehensive literature review, a better model combination method, theoretical proofs and more experiments.