222Rn (radon) is the major radioactive gas in volcanic area. The study of radon concentration variations in volcanic areas has been considered as a useful tool to investigate the volcanic activity in one area. Tatun Volcano Group, where close to Taipei basin and exhibits active fumaroles and hot springs, was chosen for first time systematical study of radon gas in Hsiao-You-Keng hydrothermal area of northern Taiwan. In addition to soil CO2 flux and gas compositions, radon concentrations in soil were measured in situ by an analogous radon sensor. The sensor is developed to separate the electronics from the measurement chamber, which contains the detector and the preamplifier completely covered by epoxy, and can be used in the aggressive environment. Combined with gas composition and helium isotopic data, we can conclude that the radon gas is mainly carried by the volcanic gases. Therefore, it is considered as a good proxy for monitoring the magma activity in this area. A monitoring station was newly setup for such purpose and the preliminary monitoring data will be presented in this study.