In order to gain more information reflecting bridge health status, with fewer sensors, a method based on artificial fish swarm algorithm (AFSA) is proposed to solve optimal sensor placement (OSP) problem. The algorithm takes modal assurance criterion (MAC) matrix obtained from modal analysis of an arch bridge structure as the objective function. Four typical behaviors of artificial fish are applied to search the optimal solution. The results show that AFSA is more effective than the particle swarm optimization (PSO) method, in achieving optimal sensor placement.