Isolation and quantitative determination of costunolide from Saussurea lappa Clarke (Compositae) has been conducted by using HPLC method. Costunolide in an acetone extract from the crude drug was separated on a RP-18 column using a $MeOH-H_20$ mixture (65:35) as an eluent and the average content is about $1.32{sim1.42%.$ The content of costunolide in dried extract was decreased by about 24% in seven days. However it showed a slight decrease in solution. It is highly recommended that quantitative determination of costunolide from Saussureae Radix should be conducted as early as possible after solvent extraction.