Abstract: To avoid isotopic fractionation during the process of volatilization, we injected the volatile organic compound standard in a quartz vacuum tube which was deeped into liquid nitrogen trap, then closed it and combusted in furnace for sample preparation. Takeing marketable puriss formic and acetic acids as isotope standards, 5 repetitions were made for each and were measured on Finnigan MAT-252 mass spectrometer. The measurement results have high repeatability. The relative standard deviations of the two samples are 0.07% (n=5) and 0.04% (n=5) respectively. Additionally，the analysis results of 5 same samples with gas chromatogram-combustion-isotope ratio mass spectrometry (GC/C/IRMS) reveal that there is no significant difference between the two methods, but the precision is less than the above mentioned. The error analysis also shows that the deviation of the analysis can be ignored, and the sealed-tube combustion method can be used to determine the δ¹³C values of VOCs standards.