In a grid connected distributed generator, the voltage variation problem at the point of common coupling(PCC) is investigated. This is one of the most frequent power quality issues for the grid connection of large amount of the input power into a weak grid. Through a simplified modeling of the distributed generator and the power network, the magnitude of PCC voltage variation is calculated using the equivalent circuit parameters and the output power of the distributed generator. In addition, the required amount of reactive power which can compensate the voltage variation is presented analytically using a vector diagram method. With the proposed compensation method in a power conditioning system(PCS) for a distributed generation, the PCC voltage variation can be minimized automatically even though the power fluctuation occurs due to the change of input power. The proposed method of the calculation and compensation of the PCC voltage variation is verified by computer simulation and experimental results.