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 temperature The 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.