Abstract: Both the urea adduction method and 5Å molecular sieves are used to separate straight chains from branched/cyclic hydrocarbons, however, the recovery of the low concentration of n-alkanes are generally low due to the complex process. Through optimizing experimental conditions of urea adduction method to separate n-alkanes, a method of Gas Chromatography/Isotope Mass Spectrometry to the determination of low concentration of n-alkanes in soil and plants was developed. Hexane and acetone are used to dissolve a saturated fraction. The mixture was added to a 2 mL urea-saturated methanol solution and allowed to stand for 48 h at 4℃. After the urea crystals were dissolved in distilled water, hexane was added into the mixture to form aqueous and organic phases. The n-alkanes were recovered by hexane in aqueous phase and by a mixture of hexane and acetone in organic phases. The recoveries of middle-and long-chain n-alkanes ranged from 79% to 104%, higer than the recoveries by the urea adduction method and 5Å molecular sieves. The precision of single carbon isotope ranged from 0.09‰ to 0.63‰ (1σ). Using the proposed method, the interferences of undivided peak and total outflow were significantly reduced, the recoveries of middle-and long-chain n-alkanes with ppm level in actual samples.