Return period is widely used for civil engineering planning, design and management. However, the climate change and human activities have broken the stationary condition of the hydrological series, which is the base for the hydrological frequency analysis. Therefore, it is necessary to develop a new method to calculate the return period of the hydrological elements concerned by people. Based on the two different concepts of return period, the four equations of calculating the return period under the stationary and non-stationary conditions have been summarized and developed while the assumption of the independence of the hydro-logical series can be satisfied. The equations of calculating the return period under the different conditions were applied in flood frequency at the Longchuan Station located on the Dongjiang River of Guangdong Province in China. The annual maximum peak discharge time series that are available from 1956 to 2009 have been taken in the return period calculation. The results of the return periods have also been discussed.