This study presents a novel capacitive-type Z-axis (out-of-plane) accelerometer implemented on SOI wafer. This accelerometer contains special designed gap-closing differential sensing electrodes. The present Z-axis accelerometer has four merits, (1) mass of the proof mass is increased by combining both device and handle silicon layers of SOI wafer, (2) the sensitivity is improved by the gap-closing differential electrodes design, (3) the electrical interconnection between the device and handle silicon layers of SOI wafer is available by means of the metal-vias, (4) the sensing gap thickness is precisely defined by the buried-oxide layer of SOI wafer, and In application, the Z-axis accelerometer is fabricated and characterized. Typical measurement results demonstrate that the present Z-axis accelerometer has a sensitivity of 196.3mV/G, a non-linearity of 2% over the range of 1G acceleration, and a resolution of 0.01G. ©2009 IEEE.