Abstract: A Preliminary design theory of hydrogensensitive Pd-gate-MOSFET is developed. By this theory the operation temperature, gate dimension, and the bulk resistivity of the Pd-MOSFET Can be determined from the monodromy of the threshold voltage, response time, actual measurement sensitivity, threshold voltage and the reliability of the devices, In addition, the structure, technology and characteristic of the Pd - MOSF ET designed and made by ourselves are reported. The threshold voltage in the air containing 1% H₂ is 600 mV lower than in pure air, Typical response time is approximately 0.5 minutes at 150℃.