Abstract: A non-derivatization liquid chromatography-mass spectrometry（LC-MS）method was established for the determination of decylic acid and octylic acid in beer. Beer samples were extracted with n-hexane at a ratio of 1∶1（V/V）, and the upper layer was dried and re-dissolved in methanol. The determination was carried out using the selected ion monitoring（SIM）mode of LC-MS. The linearity, detection limit, quantification limit, precision and recovery of decylic acid and octylic acid were investigated. The results showed that decylic acid and octylic acid could be well separated under the above chromatographic conditions. The linear relationship between peak area and concentration was good. The linear range of decylic acid was 0.1−15 μg·mL⁻¹ with r = 0.9975, and that of octylic acid was 0.4−50 μg·mL⁻¹ with r = 0.9994. The detection limit and quantification limit of decylic acid and octylic acid were both satisfactory for sample determination. The method had good precision. The average recoveries of decylic acid and octylic acid in beer were 94.5% and 98.9%（n = 9）, respectively. In conclusion, this method was validated and could be used for the determination of decylic acid and octylic acid in beer. The contents of decylic acid and octylic acid in six beer samples were determined by this method, with the content range of decylic acid being 0.2803−1.741 μg·mL⁻¹ and that of octylic acid being 1.722−4.860 μg·mL⁻¹.