In a dynamic analysis of the bridge, the damping ratio is usually set from a main material. On the other hand, the energy-based damping theory is developed in recent years. Therefore, This study concerned with energy-based estimation method of damping ratio of single span PC stress ribbon bridges. The equivalent damping ratio of the slab becomes h_G=0.0101 form the statistics analysis of the measurement damping ratio, and modal damping can be estimated form the equivalent damping ratio and the ratio of the strain energy to the vibrational energy. In addition, the influence which the damping ratio gives to the serviceability for the pedestrian-induced vibration was examined by the parameter analysis, and considered about presumption accuracy of the damping ratio technologically needed.