The author performed some experimental investigations into the unsteady cavitation of model propellers in irregular flow and vibrating blades in uniform flow. From the results of the experimental investigations and the analyses of the wake patterns, followings have been obtained : 1. the reduced frequency of the blade element of the propeller in irregular flow is one of the most important factors for the cavitation, 2. the critical reduced frequency is around 0. 30. 4, 3. genrally speaking, the critical reduced frequency for the back cavitation occurs near the tip of the propeller blade and that for the face cavitation at 0. 70. 8 R ( R ; radius of the propeller) for the single screw ship and, 4. to avoid this critical reduced frequency, the hull form as well as the propeller should be improved.