Inhibitory activity of the three major triterpenes from the stems of Cynomorium songaricum--ursolic acid, acetyl ursolic acid, and malonyl ursolic acid hemiester--and their related compounds were tested for their inhibitory activity on HCV protease; malonyl ursolic acid hemiester was the most potent. A HPLC-PAD (photo diode array detector)-MS method was established to quantify the contents of each triterpene in C. songaricum. Using this method, the effect heating had on the contents was also investigated. It was found that among the three triterpenes, the content of malonyl ursolic acid hemiester decreased most quickly during the heating process.