To study the chemical constituents of the essential oil of Cassiope selaginoides and its anticoagulant activity.The C. selaginoides essential oil (CSEO) was extracted by solvent extraction method using petroleum ether (60-90 ℃) as the extracting solvent,and the chemical constituents were analyzed by GC-MS.The anticoagulant activity of CSEO was evaluated by prothrombin time assay in vitro.The results showed that the CSEO contained 84 compounds,and 47 components accounting for 80.72% of the total oil were identified.Among them,the principal components in CSEO were ursane-and oleane-type triterpenoids,mainly friedoolean-8-en-3-yl acetate (17.85%),followed by aromatic compounds,aliphatic hydrocarbons,monoterpenoids,steroids,and rare sesquiterpenoids.In addition,prothrombin time test revealed that CSEO had an effective anticoagulant activity.