A novel radial runout measurement method for gear motors using a microsensor based on all-fiber Fabry-Perot interferometry is investigated. In order to achieve the fault diagnosis, in this method, a single-mode fiber is put forward as a sensor to measure radial runout of the rotating shaft. The performance of the proposed sensor has been compared to a Portable Digital Vibrometer-100 laser vibrometer for validation purposes, and the results show that the difference between them is approximately ±0.55µm.