This paper proposes a loose bolt detection system with battery-free wireless sensors based on backscatter communication. Loose bolt detection has been performed using wired or optical sensors. However, wired sensors demand temporary attachment of sensors and the routing of wires. Optical sensors demand precise alignment and the line of sight to the measurement points. The proposed backscatter system essentially eliminates those problems and can be used even when the target machine or structures are in their operation if the backscatter sensors are implanted to the target structure. On the other hand, the proposed system involves the challenges of data synchronization and the limited bandwidth. In this study, a prototype wireless loose bolt detection system based on transmittance function and damage index is developed and is comparatively evaluated with wired accelerometers by a series of experiments. The data synchronization problem was tackled by preprocessing of the collected data. It is also revealed that sufficient bandwidth in the wireless communication channels is important for accurate detection.