Abstract: The volatile oil of Polygonum orientale L. was extracted by hydrodistillation method, and the oil was analyzed by GC/MS. The optimal conditions of extraction was the medicinal materials being dipped in 800 mL distilled water for 4 h, and distilled 8h by steam. 24 compounds are identified. The most constituents in the oil of Polygonum orientale L. are terpenes compound(27.36%） and ketone compound(20.07%）. The main constituents are isolongifolene (19.45%), α-caryophyllene (0.84%), α-pinene(0.99%), caryophyllene oxide(3.20%) and geranylacetone (5.04%).