The two new triterpenes, named alisol A and alisol B, have been isolated from the rhizomes of Alisma Plantago-aquatica L. var. orientale SAMUELS. (A. orientale JUZEP.), and their structures were elucidated by both chemical and X-ray crystallographic techniques. Alisol A (1) was demonstrated to be a tetracyclic triterpene in which the iso-octyl side chain possesses the glycerol moiety mainly by the chemical evidences that the periodate oxidation of the compound evolved acetone together with the tetranoraldehyde (7), and that acetonization of the compound followed by acetylation afforded the acetonide monoacetate (9). The structure of the ring system was also discussed by physico-chemical measurement. The X-ray analysis on alisol A acetonide monobromoacetate (11) was carried out to establish the structure of alisol A as 1. Alisol B was correlated to alisol A, and was assigned the structure 2. This is the first occurrence of C_<30>-triterpenes having the fusidane-type B/C ring juncture together with a double bond at 13(17) position.