Abstract: n-alkanes were concentrated after separating by a 5A molecular sieve and extracted by a mixture solvent of cyclohexane and n-pentane. In this study, the amount of 5A molecular sieve and the ratios of cyclohexane and n-pentane were optimized as 2.76 g and 9∶91, respectively. It was discovered that short carbon chain n-alkanes were readily absorbed by a 5A molecular sieve, compared to the long carbon chain n-alkanes. The complexation efficiency is higher for long carbon chain n-alkanes than that of short carbon chain n-alkanes with enough 5A molecular material at higher temperature. In this study, n-alkanes contents were determined by Gas Chromatography (GC) and compound specific carbon isotope ratios of individual n-alkanes in soil were measured by Gas Chromatography-Isotope Ratio Mass Spectrometry (GC-IRMS).The recoveries of n-alkanes ranged from 44% to 72‰ with a precision of 4%-8%. The accuracies of compound specific carbon isotopes of n-alkanes ranged from 0.04‰ to 0.38‰ (1σ). The 5A molecular sieve procedure was used to analyse soil samples around a gas station. The results indicate that almost all of the unseparated and interference peaks were eliminated and the purification of n-alkanes met the requirements of stable carbon isotope analysis.