For an active vibration control of flexible structures, there is a limitation due to amount of signal that limited number of sensors can produce. Since misplacement of sensor causes the lack of system observability, it is important to optimize the locations of sensors base on observability concept. In this study, the observability grammian is acquired using the modal sensing constants, natural frequencies and eigenfunctions of plate. The eigenvalues of observability grammian matrix are used to yield the observability performance index to be optimized by Developed Optimization Algorithm (DOA) and Genetic Algorithm (GA). The effectiveness of proposed location observes the first two modes of frequency much more observable than initial sensor's location.