This paper reports a small-scale electromagnetic energy harvester with inertial rotary structure, which can convert linear motion to high-speed rotation and keeps rotating inertially. With a single compression, the rotation speed of rotor is estimated as high as 3500 rpm and it can rotate inertially as long as 50 s. The energy conversion efficiency (from rotor's kinetic energy to electric energy) during this process is 92.4%. Therefore, the device can efficiently harvest energy from ultra-low frequency (<1Hz) vibration. During the test, RMS output power of about 11 mW and 6.5 mW can be achieved under external vibration frequency of 0.84 Hz and 0.1 Hz, respectively. Finally, the energy harvester is mounted under shoes to harvest energy from human motion. Average power of 85 mW can be harvested from running at a speed of 9 km/h.