This study specifically focused on the damping ratio among structural parameters and aimed to evaluate its fluctuations due to temperature. The research investigated the stability of quantitative evaluation methods for the damping ratio. The objective was to assess annual fluctuations in the damping ratio using vibration monitoring data from bridges in service. For the widespread adoption of Bridge Health Monitoring (BHM), a bridge with small to medium spans common in Japan was selected. The focus was on practical health monitoring for road bridges, utilizing the Eigen-system Realization Algorithm (ERA) and a relatively lightweight calculation process based on the Random Decrement (RD) method. As a result, a relationship between the bridge damping ratio and temperature was demonstrated, suggesting the possibility of nonlinear behavior in response to temperature-induced changes.