Abstract: OBJECTIVE To optimize the best oxidative decomposition condition of the volatile oil from Atractylodis Macrocephalae Rhizoma(VOA), and clarify the effects of oxidation induced by UV lamps and sunlight on compositions of volatile oil. METHODS The VOA produced in Anhui was extracted by supercritical CO₂ extraction technique, and then separately treated with irradiation oxidation under UV lamps of 254 nm/8 W, 365 nm/8 W and 254 nm/30 W. The optimal oxidative decomposition condition was selected through monitoring the change of atractylon content by HPLC. The effects of UV lamp irradiation on compositions of VOA and their relative contents were analyzed by GC-MS and the compared with decomposition product through sunlight oxidation. RESULTS The decomposition rate of atractylon in descending order was 254 nm/30 W>254 nm/8 W>365 nm/8 W. The disappeared, newly appeared and content changed chemical compositions under the conditions of UV and sunlight irradiation were varied, but the overall change trends were consistent. As the oxidative decomposition of atractylon and γ-elemene, the contents of five anti-tumor components including isovelleral, β-eudesmol, costunolide, atractylenolide Ⅰ and atractylenolide Ⅲ significantly increased. The contents of isovelleral and β-eudesmol were higher after oxidated by sunlight, while the contents of compositions of lactone class were higher after oxidated by UV lamps. CONCLUSION It is a good method to solve the stability of VOA by oxidating under 254 nm/30 W UV lamp. However, the cost of oxidation induced by sunlight is lower if considering pharmaceutical scale.