To increase the efficiency of photovoltaic (PV) systems, maximum power point tracking of the solar arrays is needed. Output power of solar arrays depends on the solar irradiance, temperature and load. Irradiance changing effects on current is more than voltage and with irradiance increasing, current increases more than voltage and finally power increases. But effect of temperature changing is more on voltage and with temperature increasing, voltage decreases and in result the power decreases. If solar array be under shade conditions, it's P-I characteristic has multiple peaks and finding of real maximum power point is a problem. This paper presents a fast maximum power point tracking with improved Particle Swarm Optimizationfor PV systems under normal and partially shaded conditions. Simulation results confirm that proposed MPPT algorithm and kinds of it with high accuracy can track the peak power point under different irradiation, temperature and partially shaded conditions and they have results near to real value.