A method of derivatization at injection port following three phase hollow fiber microextraction with tetramethylammonium hydroxide (TMAH) as a dual-function reagent, for the simultaneous determination of salicylic acid, p-hydroxybenzoic acid, cinnamic acid, vanillic acid, p-coumaric acid and ferulic acid in beers by gas chromatography was developed. Phenolic acids were extracted from aqueous samples to a thin layer of organic solvent ( hexyl acetate) phase impregnated into the pores of the hollow fiber wall, and then back extracted to an acceptor solution (TMAH) located inside the lumen of the hollow fiber. Upon injection, the phenolic acids were derivatized to their methyl esters in the GC injection port. Several parameters related to the derivatization and extraction efficiency were optimized. The optimized conditions were as follows: hexyl acetate was used as the extraction solvent, an aqueous solution of TMAH (5% W/ V) was used as the derivatization reagent and acceptor phase, the pH value of donor phase was 2. 0, the concentrations of NaCl was 25% W/ V, the stirring rate was 500 r/ min, the extraction time was 40 min. Under the optimal conditions, the linear range of phenolic acids was 0. 50 -15. 00 mg / L, the limits of detection were 0. 05 -0. 18 mg / L. The proposed method was applied to the determination of the phenolic acids in beers. Vanillic acid, p-coumaric acid and ferulic acid were found in the beer samples, others were not detected. The spiked average recoveries were 90. 1% -106. 8% and RSD% was less than 5. 9% (n =3). The method is suitable for the determination of phenolic acids in beers.