A new method for PID controller tuning based on process models for cascaded control systems is proposed in this paper. The method consists of first finding the ideal controller that gives the desired closed loop response and then finding the PID approximation of the ideal controller by Maclaurin series. This method can be applied to any open loop stable processes. Furthermore, it enables us to tune the PID controllers both for the inner loop and the outer loop simultaneously while existing tuning methods tune the inner loop first and the outer loop next. Closed loop responses of cascade control loops tuned by the proposed method are compared with those of existing methods such as the frequency response method and the ITAE method. The results show that the proposed tuning method is superior to the existing methods.