Abstract: The measuring method of rare gas isotope raios using the mass spectrometers developed by author for geology research and earthquake monitor is reported in the paper. We have measured 3He/4He and 4He/(20)Ne in Tibet Yangbajing Subterranean field and Changbaishan volcano field. The source of escaped rare gas in the two fields is researched. In addition, the continuouse monitor of 4He/(20) Ne. 4He/(40) Ar and (40)Ar/(36)Ar ratios in Taryun soil gas is carried out in order to study earthquake forecast. Some earthquake examples are obtained.