Decision Tree approach for security assessment and evaluation is presented. The general decision tree methodology is proposed for the purpose of predicting the robustness of a power system in the occurrence of severe disturbances, and of discovering appropriate control actions whenever needed. The decision trees are built off-line and are used on-line. Their building calls upon an inductive inference method, which automatically extracts the relevant information from large bodies of simulation data and express in terms of the system controllable variables. In this paper Decision trees are used for classifying the operating states of the power systems as either secure/insecure using the MW flows and voltages and the severity level of the violation and the control actions necessary by finding the location of operating point when dropped at the top node of the decision tree. © 2005 IEEE.