Abstract: The progress of oil gas geochemistry has been accelerated by the development of determination on argon isotopic composition in recent years. The main difficulty with determination on argon isotopic composition in natural gas is the presence of atmospheric argon as a ubiquitous contaminant. This problem can be partially alleviated by the high vacuum purification and separation system designed in this work. On the request of “973” reseach project, some natural gas samples determined by ZHT1301 and MAT 271 mass spectrometry ten years ago are measured. By comparison between the new data and old determinations, it is found that the old measurement was contaminated by the high background and the method of sample transferred with the isotopic fractionation and about 73% of the old data can be used to calculate the source rock age. During determination of argon isotope in this work, a method called “transferring sample under tranquil state by different intervals” is used for avoiding the isotopic fractionation during the moving of sample to analytical room.