Core technologies for integrating hydrogen gas sensor were investigated. In this study, the thermally isolated micro-hot-plate with areas of $100{ imes}100-260{ imes}260{mu}m^2$ was fabricated by utilizing surface micromachining technique that provides better manufacturing yield than bulk micromachining counterpart. The optimum design of the sensor was peformed by analyzing the thermal profile of the structure obtained from a ANSYS simulator. The 400-nm-thick polysilicon films doped with phosphorus, the 300-nm-thick aluminum films, and the 200-nm-thick $SnO_2$(or ZnO)films were used as the micro-heater material, the temperature sensor material, and the gas sensitive material, respectively. The experimental results show that the developed gas sensors can detect $H_2$ concentration as low as 1 ppm.