In this paper, decision tree (DT)-based preventive and corrective control methods are proposed to enhance the dynamic security of power systems against the credible contingencies causing transient instabilities. Preventive and corrective controls such as generation rescheduling and load shedding schemes, respectively, are developed based on the security regions and boundaries that are calculated in the space of appropriate decision variables. The security regions and boundaries are determined by the rules of DTs that are developed by the generated knowledge bases. This work also involves improving the accuracy of security boundaries as well as the optimal solutions for the fuel cost and load shedding optimization problems encountered in the preventive and corrective controls. The methods are implemented on the Entergy power system model.