A new highly sensitive HPLC method for determination γ-aminobutyric acid(GABA) in serum was developed by precolumn derivatization of GABA with 9-fluorenylmethyl chlorormate(FMOC-Cl) using ε-amino-n-caproic acid as an internal standard. The column used was Shim-Pack CLC-ODS(M), 4.6 mm×150 mm,5 μm. Mobile phase A consisted of sodium acetate buffer (0.05 mol/L,pH 3.65)/water/tetranhydrofunan/glacial acetic acid(250/100/15/2.2).Mobile phase B was acetonitrile/methanol (4/1). The conditions of derivatization and chromatography were studied to find a optimal procedure,which included pH value,reaction time, ionic strength and amount of derivatizing reagent.The coefficient of variance of the method was less than 4.6% for within runs, 6.1% for between runs, respectively. The minimal detection limit (S/N=2) was 3.1 nmol/L. The linearity was observed from 10 to 1 000 nmol/L, and the coefficient of determination was 0.9992. The average recovery was 97.1%.