There are several papers concerned with vibration analysis of rectangular plates without stiffeners subject to inplane loads. In this paper the authers studied the vibration of reinforced rectangular plates subject to inplane loads. And they found that the frequency of a reinforced rectangular plate under uniform compression, pure bending or pure uniform shear is less than that under no in-plane loads as well as in case of plates without stiffeners. The analysis of plates with reinforcement under any boundary conditions can be treated simply and generally by this method.