In this study, a fractional-order PID (FOPID) controller is designed to control a DC-DC boost converter in a PV-system. Because of the nonlinear V-I characteristic of a PV-panel, a power electronic interface is required to obtain a desired and fixed voltage level. In order to obtain the best system performance, parameters of the proposed controller are tuned by using Particle Swarm Optimization (PSO) algorithm. Both of the system responses with the FOPID and classical PID are tested under various power conditions by changing the load resistor and solar irradiation values. The simulation results are compared in terms of integral of time weighted squared error (ITSE) criterion, percentage overshoot (M-p) and rising time (T-r). The results show that the FOPID controller performs better performance than the classical PID controller.