Nowadays, the energy harvesting has attracted considerable interest of international researchers from many disciplines, due to the promising application around the industrial area. The energy harvesting technology is very important because it can extract electrical energy from ambient environment for supplying power to wireless monitoring nodes. Although much work has been done for extracting energy from vibration motions, there are few successful applications for the rotational motions. Also, those energy harvesters for rotational motion will be incapable for realistic applications when the priori knowledge of monitoring objectives is absent. This paper proposes a novel electromagnetic energy harvester for the online bearing health condition monitoring. The circular Halbach array is introduced to the arrangement of permanent magnets, which can achieve frequency-up conversion and enhance the magnetic field. In order to optimize output power performance, the theoretical magnetic field model to analyze the magnetic coupling is established. The proposed dynamic model can predict voltage response and output power. The numerical and experimental results show that the output power and power density can be improved for condition monitoring.