The Gas Chromatography has been applied to the direct determination of volatile fattyacids (VFAs, ethanol, acetic acid, propionic acid, butyric acid, pentanoic acid, caproic acid and octanoic acid) in the anaerobic fermentation liquid of synthesis gas. Two gas chromatographic conditions of order temperature hoist and constant temperature were investigated, followed by acidification of the sample（pH=3）and GC analysis，with PEG-20M stationary liquid capillary column and flame ionization detector（FID）. The optimum conditions（constant temperature）including the detection limits, precisions and recoveries have been analyzed. The results show that under constant temperature,the total time for separating VFAs including ethanol,acetic acid,propionic acid,butyric acid,pentanoic acid and caproic acid is about 5 minutes but the time for separating octanoic acid is 9.8 minutes.The linear range of VFAs were good linearity with the correlation coefficients r≥0.993 7, the detection limits were in range of 0.4—1.1mg/L. The recoveries of the actual sample with different concentrations were between 94.5—109.3% and the relative standard deviations were less than 10.4%.