In active vibration control of a smart flexible structure, the placement of actuators and sensors is a key aspect of the control system design. The optimal placement of piezoelectric actuators and sensors using modal H2 norm criteria is considered. Based on Rayleigh-Ritz and modal theory, the system modal equations and closed-loop state space equations are derived. Then, a synthetic modal H2 norm criterion representing the controllability and observability of the system is proposed. Using an improved genetic algorithm, the optimal configurations of collocated piezoelectric actuators and stress sensors of the system for single and synthetic modals are found. To verify the validity and feasibility of the presented optimal criterion and method, an experimental system is designed and set up. The analytical and experimental results demonstrate that the smart flexible beam system has a high degree of controllability and observability, such that both the single modal of interest and synthetic modals are well observed and controlled. The proposed optimal criterion and method are feasible and effective.