A simple RP-HPLC method with UV detector at 270 nm was developed and validated for quantitative gallic acid determination. the optimum condition for determining were also investigated. Separation was performed on a Omnispher C18 (250mm×4.6mm×5µm) using mobile phase comprising of Methanol : Water : 0.1%Ortho phosphoric acid (20 : 79.9 : 0.1 v/v) at a flow rate of 1.0 mLmin-1 isocratic elution mode. Linear relationship was found to be in the range of 1-200 mgL-1 for gallic acid. Regression equation showed good calibration curve (R2=0.9998) between the peak are of gallic acid and concentration. The limit of detection (LOD) and limit of quantification (LOQ) were 0.023 and 0.078 mgL-1, respectively. The relative standard deviation was found to be in the range of 4.55, 0.89, 1.18% for 1, 10, 30 mgL-1 gallic acid (n=13). The proposed method was successfully applied to the determination of gallic acid in longan sample; E-dor, E-dang and Sichompoo.