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In the industry, many plants are described by higher order systems. Most of the time, higher order systems are approximated with the lower order systemusing model reduction method, and then the appropriate controllers are designed. In this paper, the controller for higher order system is designed without using model reduction methods. Instead, a fractional PID (FPID) controller is designed for higher order system. In simulations, ten different plants were examined, ranging from order 3 to order 7, with and without delay. The time-domain optimal tuning of higher order systems was carried out using integrated squared error (ISE) as the performance index. Results indicate that the controller for higher order system can be designed without model reduction methods by using FPID controller. The results of FPID controllers are also compared with classical PID controller. The FPID controller displayed robust performance; better gain and phase margin. The complementary sensitivity and sensitivity functions are better achieved with FPID controller. The FPID controller exhibits an iso-damping property (flat response around gain crossover frequency) for higher order systems.

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