The paper investigates the effects of the dimensionless number flooding number that can exactly predict the flooding phenomenon in the cathode channel. Experiments were carried out at 0, 50, and 90% relative humidity and at 50 and 60 °C cell temperature respectively. The experiment gave a dimensionless number that could effectively predict the flooding effect in the cathode channel of the PEM fuel cell. As the study was performed the factors such as temperature and relative humidity that influenced the dimensionless number, the results verified the dimensionless number. These results have been connected to the cathode channel flooding. The influences of outlet temperature and relative humidity on flooding at different levels of current density are discussed.