Abstract: The hydrogen isotopic compositions in single hydrocarbons was measured by gas chromatography-thermoconversion-mass spectrometry (GC/TC/MS). The results showed that the temperature in the reactor, the gas amount injected into GC and the split ratio of natural gas injected into GC inlet affect the determination result of the hydrogen isotopic composition. Using the certified reference natural gas, the hydrogen isotopic of methane in Wen31 well is δD v-smow= -154‰. The uncertainties of the hydrogen isotopic measurement of methane, ethane and propane were researched. Although their measuring standard deviation is less, the uncertainty of results is larger because of the larger uncertainty of the certified reference natural gas.