The need to predict cavitation behavior in a hydraulic turbine is one of the most important problem for the hydraulic community. In this paper, numerical method for predicting critical cavitation performance in an axial hydraulic turbine is presented. In this method, unsteady cavitating flows around the axial hydraulic turbine runner is calculated to apply bubble two-phase flow model. Results compared to the experiment show a good agreement for the volume of cavity and the performance drop off.