In this paper, based on the optimal placement of sensors and actuators, the vibration control by using a system of modal sensor and modal actuator with a small number of sensors and actuators is realized for a plate structure. The modal sensor consisting of accelerometers as well as the modal actuator of lead zirconate titanate (PZT) is built up for a CFRP cantilevered plate. The structural vibration control is realized by the independent modal space control based on the linear quadratic regulator (LQR) control theory. Sensors and actuators are optimally placed so that the best accuracy of measurement of modal velocity and the maximum control effect can be acquired. From the numerical and experimental results, it is demonstrated that the optimal placement of sensors and actuators is very important to stabilize a control system when the number of sensors/actuators is limited, and the vibration of plate can be suppressed by the state feedback control for each mode using the modal sensor and actuator optimally designed.