The unsteady rotating force or dipole strength distribution, acting by the fan or propeller on the fluid, is predicted by inverse method. In this method, the far-field acoustic pressures are used in non-cavitating condition. In this paper, the far-field acoustic pressures are obtained from Ffowcs Williams and Hawkings (FW-H) equations using computational fluid dynamic (CFD) in specific hydrophone array and then the unsteady rotating force, acting by the propeller on the fluid, is obtained as the most important sound source in non-cavitating condition. The unsteady rotating forces are extracted using inverse method by analytical code in MATLAB. The correct solution is independence to the optimum select of regularization parameter from transfer function; the transfer function represents relationship between the force coefficients and the far-field acoustic pressure. Therefore, the appropriate range of regularization parameter should be choice in order to an ill-conditioned problem from transfer function is solve. The analytical code is solved for different regularization parameters and then the unsteady rotating forces are obtained for three sections on the blade surface. The inverse method could be used for dipole strength distribution calculation as the most important sound source in non-cavitating condition in order to design the noiseless of marine propeller.