Although application number of polymer insulators increases year by year, standard test methods for polymer insulators have not been established yet and various test methods are proposed all over the world. Generally polymer insulators have superior surface hydrophobicity and they show higher contamination withstand voltages than conventional porcelain insulators. It is reported that under some rain conditions, their contamination withstand voltages were lower than those under fog conditions. Therefore, we made contamination withstand voltage tests of polymer insulators under various simulated rain conditions in order to find out the most critical wet condition for these polymer insulators.