The changes of contents of dehydrocostuslactone (I) and costunolide (II) in the extract of Saussureae Radix were investigated during decoction. The contents of I and II in infused solution were reached to maximum after about 10min of the decoction, respectively. However, when the decoction was continued until the volume becomes to half, the contents of I and II in the solution decreased significantly compared to maximum content in the decoction. The content of II also decreased rapidly during the reflux in water. These results indicated that these small availabilities cause not only by volatilization but also by decomposition.