This paper reports a new MEMS-based chemical sensor for hydrogen gas detection, using a highly sensitive micro-thermal sensor. The proposed micro-thermal sensor detects the reaction heat between novel metal catalyst and hydrogen by measuring dc voltage from Cr-Bi thermopile. The proposed micro-thermal sensor, which consists of reference and sensing thermopile, was fabricated on a high thermal resistivity layer for reducing parasitic heat transfer. A metal catalyst was deposited on only the sensing thermopile. And then, In order to remove noises such as external change of temperature, gas flow and electrical noise, a difference between output signals of reference and sensing thermopile was measured by using differential amplifier. To verify its feasibility of the sensor for hydrogen gas detection, various concentrations (0.1, 0.5, 1%) of hydrogen gas were measured. The result of the measurement shows that output voltages of the sensor increases linearly according to the gas concentrations.