This paper shows a performance of abnormality detection of gear systems using Micro Electric Mechanical Systems Acoustic Emission sensors (MEMS-AE sensors), which are newly developed to be able to install in gear systems having only narrow installation spaces. To determine the performance of the MEMS-AE sensors, both the MEMS-AE sensor and a conventional AE sensor have been applied to a power-circulating-type gear test machine, and fatigue tests of metal gears have been performed, where the property of the MEMS-AE sensor and the conventional AE sensor have been compared. As a result, we have found that the AE count rate with the MEMS-AE sensor increases as the gear failure proceeds, especially, this AE count rate begins to increase before small pits occur on surface of the test gear. This property is the same as that of the conventional AE sensor, in other words, MEMS-AE sensor can sufficiently detect the abnormality of metal gears equaling the conventional AE sensor.