In this paper, the behavior of copper and steel rectangular plates with clamped boundary conditions subjected to underwater explosion loading is investigated. Cavitation is a phenomenon that occurs in this process. During the cavitation, the total pressure of the explosion becomes zero, so that the governing equations of motion time will be different before and after the cavitation. As a result, in terms of analysis and design, the cavitation time is significant in studying the behavior of a rectangular plate at underwater explosive loading. To calculate the cavitation time, the equations of motion of a rectangular plate underwater explosive loading are derived first, based on Hamilton principle and variation method. Then, in order to obtain the forced response of the rectangular plate, the exact free vibration solution of the rectangular plate is derived for exact mode shapes. Then, the speed and generated stress of plate during cavitation time are calculated and compared with the yield stress of copper and steel rectangular plates. Using this method, one can distinguish the cavitation with in the elastic or plastic regimes. Results show that the cavitation time is on the order of microsecond.