Environmental factors such as dust adsorption effects and humidity generally result in drifts of the sensor output signal and lead tomalfunctions of the sensor and/or reduction in sensitivity. So a sensor package is necessary to protect hydrogen sensors in order to operate the sensors in an actual use environment. In this study, a Pt-FET hydrogen sensor covered with a polytetrafluoroethylene (PTFE) porous membrane was developed. The PTFE porous membrane with different pore sizes of 5 μm, 20 μm A and 30 μm was applied and the effects of humidity on sensor sensitivity were evaluated. The results indicate the effects of humidity on the sensitivity were reduced in the relative humidity range between 20% and 80% for the sensor with the 5-μm porous membrane. © 2010 The Institute of Electrical Engineers of Japan.