Abstract: HPLC-MS/MS was applied to the determination of 4-nonyl phenol (4-NP) in edible vegetable oils. The sample of edible vegetable oil was dissolved in mixture of ethyl acetate-cyclohexane (1+1),and then purified by gel permeation chromatography, through collecting the fractions between 14.0 to 17.0 min, and the solution obtained was evaporated to near dryness at 30℃. The residue was taken up and diluted to 1.0 mL with methanol. In HPLC separation, Agilent Poroshell120ec-C₁₈ chromatographic column was used as stationary phase, and the mixture of methanol and 0.1% (mass fraction) ammonia solution mixed in different ratios was used as mobile phase in gradient elution. In MS/MS determination, ESI^- and multi-reactions monitor mode were adopted. 4-n-NP-D₄ was used as internal standard and blank corn oil was used as matrix. Linearity range of 4-NP was found between 0.25 μg·kg^-1 and 160 μg·kg^-1 with detection limit (3S/N) of 1.00 μg·kg^-1. Tests for recovery were made by standard addition method at 3 concentration levels of 2.00, 5.00, 10.00 μg·kg^-1, giving values of recovery and RSD's (n=6) in the ranges of 99.0% to 115% and 4.3% to 8.1% respectively.