This paper analyzes the THD(Total Harmonics Distortion) according to output power fluctuation of photovoltaic(PV) systems using real time simulator. For being close to reality, the PV system including inverter and MPPT(Maximum Power Point Tracking) which composed of closed-loop using real-time simulator are modeled. The change of irradiance and temperature of PV module is modeled to consider the change of weather. The various simulations according to the weather conditions are performed and THD is calculated at each condition. In this paper, the results by off-line simulation are compared with the ones of real time simulation.