In this work, we have designed, characterized, and optimized an inertial-based rotational vibration energy harvester device which can effectively harvest energy from vibrations at ultra-low frequency (0.02 Hz). The device is mainly composed of a twist-driving system, a pawl-ratchet clutch system, and a magnet-coil transduction system. With the twist-driving system, the linear motion from vibration can be converted to high-speed rotation of the disk. The pawl-ratchet clutch system is designed to deliver and store the kinetic energy during vibration and keep rotating inertially during the interval between two impacts. The magnet-coil transduction system is applied to transfer the kinetic energy to electricity. The device can achieve a high maximum energy conversion efficiency of 97.2% with a single compression. A high power of 6 mW has been harvested when the device is excited by the continuous compression at low frequency of 0.1 Hz. We have also demonstrated a few applications for this device, which can be mounted under the shoes to harvest energy from human motion as regular power supply. An average power of 85 mW has been successfully harvested with the running speed of 9 km/h. The rotation speed of the rotor can reach 140 rps, and the mean energy conversion efficiency can be as high as 86.81%. Furthermore, a humid-temperature sensor, and 258 LEDs can also be powered by human motion at a walking speed of 3 km/h. Finally, the device has been employed as a self-powered vehicle speed sensor, showing promising application on field.