Sizing and placement of piezoelectric sensors and actuators (S/As) for active vibration control of flexible structures is commonly based on maximum control effectiveness. Mounting of piezos changes the natural frequencies Of existing structure, Which Would have been designed to have a certain natural frequency spectrum in relation to the disturbance excitation. In the event of failure of the active system. natural frequencies of the structure with piezos (now rendered passive) become significant. Therefore minimal change in natural frequency along with good controllability should be design consideration while deciding sizing and placement of actuators. Multi-objective genetic algorithm is shown to be useful for optimization of placement and sizing of piezo actuators for such problems. Optimal S/As sizing and locations have been found out for stationary and rotating cantilever beams with multiple sensors/actuators.