© 2019 American Society of Civil Engineers. Because fatigue damage considerably reduces the service life of steel bridges, maintenance for fatigue damage should be performed properly. For the effective maintenance of fatigue damage, it is important to monitor traffic loads over long periods of time because fatigue damage is caused by repeated high stresses due to the weight of traveling vehicles. This study verified the long-term effectiveness of a portable weigh-in-motion (pBWIM) system through 1-year field measurements conducted on an actual in-service bridge. The pBWIM consists only of microelectromechanical systems (MEMS) accelerometers and MEMS inertial measurement units (IMUs), which are ideal for field measurements because they can be easily attached and detached using a magnetic jig. Furthermore, because MEMS accelerometers and MEMS IMUs are low-power-consumption devices, they are suitable for long-term field measurements. Field measurements were performed using a test truck in winter, summer, and autumn. The results indicate that regardless of the ambient temperature, the pBWIM system can estimate gross vehicle weights (GVWs) within an error of ±10.6% over a 1-year period compared with static weights measured using a truck weighing scale. Furthermore, it is found to be important to determine the dependence of the influence line on ambient temperature to estimate GVWs with high accuracy.