This paper describes a computational method to predict the propeller performance under the supercavitating condition. The present calculation is based on a vortex lattice method. Constant pressure condition on the mean camber surface in the cavitating region is satisfied by reforming the mean camber surface iteratively according to the change of load distribution due to cavitation. Calculated results on the propeller force and the efficiency of two supercavitating propellers showed good agreements with experimental ones.