The metabolic transformation of (-)-clausenamide, isolated from the leaves of Clausena lansium (Lour.) Skeels, was studied in vitro with phenobarbital-induced rat liver microsomal incubate containing the NADPH-generating system. The constitution of the incubation system was optimized and a RP-HPLC-DAD method was developed for the on-line analysis of (-)-clausenamide and its metabolites. The major metabolites were isolated and purified by silica gel column chromatography, preparative TLC and HPLC and their structures were determined as 6-OH clausenamide (CM1) and 5-OH clausenamide (CM2) by 1HNMR and MS.