Supercritical carbon dioxide, with and without a methanol modifier, was used to extract sinomenine from Sinomenium acutum (Thumb) Rehd et Wils. Sinomenine determinations were carried out using high-performance liquid chromatography (HPLC). The results show that the yield obtained after 2.5 h extraction with methanol-modified supercritical carbon dioxide was the highest (7.47 mg/g), while that obtained with only supercritical carbon dioxide was the lowest (0.17 mg/g). The recovery obtained with supercritical carbon dioxide, with and without a methanol modifier, could not be increased greatly by the method of the alkalinization of sample. Higher recoveries were obtained than extraction using methanol in Soxhlet extractor.