Abstract: OBJECTIVE To investigate the metabolites of caffeic acid (CA) in rats. METHODS After an oral administration of CA (50 mg·kg-1) to Wistar rats, 0-4 h rats urine was collected, and metabolites of CA in rats was determined by liquid chromatography tandem multi-stage mass spectrometry (LC/MSn). RESULTS The results revealed that two methylated derivatives (ferulic and isoferulic acid) of CA, two CA-glucuronides, two CA-sulfates, one CA-di-glucuronide, two methylated CA-glucuronides, two methylated CA-sulfates were found in rats urine. CONCLUSION CA was extensively metabolized in rats and structures of the metabolites will be further confirmed.