Abstarct: This study aimed to determine the flushing time, by using numerical model Mike 21, and comparison with the analysis method and selection the appropriate analytical method has been discussed in Chabahar Bay. In order to ensure the model, the results, calibrated with the data in Chabahar Bay, Then, for model verification, the model results have been compared with the results of the coherence software. In this study, factors such as the effect of wind, changing seasons, the effect of dispersion coefficient, the effect of Chabahar Bay ports Development Project and the effect of return flow factor were investigated and significant results were obtained. By using hydrodynamic model MIKE 21, the direction and speed of Chabahar Bay were determined. According to the hydrodynamic model outputs, the current in Chabahar Bay is from the East to the West. The results of the impact of mentioned factors indicate that the effects of wind-induced in Chabahar Bay is 13.8 percent, that compared with tidemark is insignificant value. summer is the best season for Chabahar Bay Flushing that Bay pollution removed from the bay after 16.64 day. The fall season for 19.16 day, in winter 17.64 day, in spring 17.81 day it takes until the pollution is removed. Critical areas for flushing in Chabahar Bay, was determined at all this season. If the solubility of pollution be taken from it with zero dispersion coefficient, 15% will be added to the flushing time. Development Project of Chabahar Bay ports increased the Flushing time, so that the pollution remains in the development of port troughs. The analytical method used in this study is tidal Prism method, that because of ignoring return flow to the bay during the flood, the Flushing time is underestimates. The exact method for determining the flushing time is tidal prism modified by Sanford and then compared this method with the numerical method of Mike. return flow factor Number was obtained 0.92.