An HPLC-ESI-MS method has been developed to identify and quantify two main tetracyclic triterpenes, alisol B 23-acetate and alisol C 23-acetate in the Alismatis Rhizoma (Taeg-Sa). The relative distribution of the two triterpenes in the methanolic extract of commercially available Alismatis Rhizoma was established by selective ion monitoring (SIM) mode via electrospray ionization (ESI) source. Regression equations revealed good linear relationship, and the correlation coefficients were 0.999 and 0.998 for alisol B 23-acetate and alisol C 23-acetate, respectively, between the peak areas of the components and their concentration in a range of $0.06-2.0{mu}g/mL$. It was found that there were significant differences in the amount of alisol B 23-acetate and alisol C 23-acetate between Korean and Chinese origins. The results showed that this method could be used to identify the two components in Alismatis Rhizoma with high sensitivity and selectivity.