This paper proposes the theory and application of a new designing method of the Fractional Order PID (FOPID) controller for fractional order Buck converter. First, the mathematical model and the state averaged model of the fractional order Buck converter in continuous conduction mode operation are established based on fractional calculus and the fact that the inductor and the capacitor are fractional in nature. And their bode diagrams and step responses are analyzed, respectively. It is found that the frequency characteristic of the fractional order Buck converter is influenced by the fractional orders of the capacitor and the inductor. Secondly, a FOPID controller is designed to control fractional order Buck converter in this study. In order to obtain the best system performance, parameters of the proposed controller are turned by using genetic algorithm. The system responses with the FOPID controlling is tested under different operating conditions by changing the load resistor, the input voltage, and the reference voltage values in order to evaluate system performance. The results show that the FOPID controller performs better performance than the classical PID controller in the dynamic response performance and steady-stage error regulating characteristics. And the proposed control strategy for fractional order Buck converter is strongly robust to the disturbance of input voltage and road.