This paper investigates optimal locations of piezoelectric actuators and sensors on a thin plate. To locate actuators and sensors properly is important in controlling modal vibrations well. A piezoelectric sensitivity index is introduced to select optimal locations for vibration control of each mode. The sensitivity expresses the efficiency of actuating and sensing modal forces according to locations of a piezoelectric material on a plate. The piezoelectric sensitivities for two types of plate, an all-clamped plate, and a free-free plate, are derived theoretically and are verified experimentally. Also, its usefulness Is experimentally shown to control vibration of the all-clamped plate with piezoelectric materials.