After impounding the dam, the diversion tunnel is completely affected by the reservoir normal water level. So it is necessary to predict the water inflow into the tunnel. In this research, water inflow into Azad diversion tunnel was estimated using the analytical and numerical methods for both with and without grouting ring around the tunnel. Observed water inflow has been used to validate the estimations. The general variation of the analytical and numerical results along tunnel route is alike. But, there are some significant discrepancies between numerical and analytical results based on the model geometry and boundary conditions. The average of seepage into the tunnel calculated using analytical methods is around 161.70 lit/sec for 496 m length of tunnel that has been reduced to the 11.2 lit/sec after the operation of grouting ring with 4 m diameter. Numerical modelling results show that 37.8 lit/sec water seepage will be reduced to the 22 lit/sec after modelling the grouting ring. The comparison between predictions and observed water seepage into the tunnel (18 it/sec) shows that the numerical estimation is relatively closer to the observed flow. In the analytical methods, the Rosengren method with 28.66 % of error and Vagverket method with 36.21% of error are the best and the NFF method with 94.44% of error has the maximums error.