Abstract: OBJECTIVE To study the chemical constituents by GC-MS and antioxidant activities of petroleum ether extract of Hypericum patulum Thumb. under ultrasound irradiation. METHODS The chemical constituents of Hypericum patulum Thumb. were extracted with petroleum ether using the ultrasonic extraction method and analyzed by GC-MS. Furthermore, the antioxidant activities of petroleum ether extract were measured by DPPH and ABTS^·+ radicals scavenging activity assays. RESULTS The 24 compounds were identified from the petroleum ether extract of Hypericum patulum Thumb. under ultrasound irradiation, 3-methyl heptane with relative content of 26.329% was the most among them. The types of detected compounds were alkane(57.99%), terpenoids(17.22%), aromatics(7.78%) and others(17.01%). The DPPH radical scavenging effect and ABTS^·+ radical scavenging effect were individually in logarithm relation with concentration in a certain range. The IC₅₀ value of DPPH radical scavenging was >20 mg·mL^-1, the TEAC value of ABTS^·+ radical scavenging effect was (3.71±0.1)μmol·L^-1. CONCLUSION The liposoluble constituents of Hypericum patulum Thumb. are abundant, and show certain antioxidant activity, which offer a scientific basis for further chemical study and utilization of Hypericum patulum Thumb..