AUC (Area Under the Curve) of ROC (Receiver Operating Characteristics) has been recently used as a measure for ranking performance of learning algorithms. In this paper, we present a novel probability estimation algorithm that improves the AUC value of decision trees. Instead of estimating the probability at the single leaf where the example falls into, our method averages probability estimates from all leaves of the tree. The contribution of each leaf is determined by the deviation in attribute values from the root to the leaf. We design empirical experiments to verify that our new algorithm outperforms C4.5 and its recent improvement C4.4 in AUC. Even though C4.4 with bagging outperforms our method in AUC, our method produces a single tree with interpretable results.