The objective of this paper is to isolate and determine gallic acid in stem and branch of Rhodomyrtus tomentosa (Ait.) Hassk.Gallic acid was extracted by 80% ethanol, isolated and purified by column chromatog., and recrystallized from the stem and branch of Rhodomyrtus tomentosa (Ait.) Hassk.A HPLC method was set up to determine the content of gallic acid in Rhodomyrtus tomentosa (Ait.) Hassk., using Lichrospher C18 (4.6 mm X 250 mm, 5 μm) column, and methanol-0.2%-phosphoric acid water solution (1.5:98.5) as the mobile phase, the UV detection wavelength was at 270 nm, with the flow rate of 1.0 mL/min at room temperatureThe results showed that the a good linear was obtained in the range of 0.1-0.6 μg and the average recovery was 100.6% (RSD =1.32%, n = 6).It was concluded that the method is simple with a good reproducibility for the determination of gallic acid in Rhodomyrtus tomentosa (Ait.) Hassk.