One hundred natural gas samples of different geological periods collected from variouslocalities were determined for their nitrogen,helium and argon contents as well as the isotopiccomposition of argon.The ratio Ar(40)/Ar~(36)was found to be from 305 to 9,255.A current helium-argon chronological formula for calculating natural gas ages was corrected.Instead,based uponthe chronologieal accumulating effect of radiogenic Ar~(40),an Ar~(40)-Ar~(39)empirical formula for cal-culating natural gas ages has been proposed.It can be seen from the ratio of nitrogen to atmospheric argon that most of the nitrogen inthese natural gases could not be derived from atmospheric nitrogen.The range of radiogenic He/Ar~(40)is from 5.19 to 23.18,and that for most samples from7 to 14.The He/Ar~(40)ratios of Sinian samples are from 5.39 to 6.14,indicating their derivationfrom the deep interior of the earth.It should be noted that the Ar~(40)/Ar~(36)ratio shows a general trend of approaching theatmospheric value from the Sinian to the Tertiary period,or from old to young in geologicalage,mirroring,to a great extent,the evolutionary tendency of the rare gas isotopic composi-tion of natural gases with continuous degassing of the earth. One hundred natural gas samples of different geological periods collected from various localities were determined for their nitrogen, helium and argon contents as well as the isotopic composition of argon. The ratio Ar (40) / Ar ~ (36) was found to be from 305 to 9, 255 . A current helium-argon chronological formula for calculating natural gas ages was corrected. Instead, based uponthe chronologieal accumulating effect of radiogenic Ar ~ (40), an Ar ~ (40) -Ar ~ (39) empirical formula for cal- culating natural gas ages has been proposed. From the ratio of nitrogen to atmospheric argon that most of the nitrogen inthese natural gases could not be derived from atmospheric nitrogen. The range of radiogenic He / Ar ~ (40) is from 5.19 to 23.18 , and that for most samples from 7 to 14. The He / Ar ~ (40) ratios of Sinian samples are from 5.39 to 6.14, indicating their derivation from the deep interior of the earth. It should be noted that the Ar ~ (40) / Ar ~ (36) ratio shows a general trend of approaching theatmospheric value from the Sinian to the Tertiary period, or from old to young in geologicalage, mirroring, to a great extent, the evolutionary tendency of the rare gas isotopic composi-tion of natural gases with continuous degassing of the earth.