The effects of green tea catechins on the main drug-metabolizing enzymatic system, cytochrome P450 (CYP), have not been fully elucidated. The objective of the present study was to evaluate the effects of green tea extract (GTE, total catechins 86.5%, w/w) and (-)-epigallocatechin-3-gallate (EGCG) on the activities of CYP2B6, CYP2C8, CYP2C19, CYP2D6 and CYP3A in vitro, using pooled human liver and intestinal microsomes. Bupropion hydroxylation, amodiaquine N-deethylation, (S)-mephenytoin 4'-hydroxylation, dextromethorphan O-demethylation and midazolam 1'-hydroxylation were assessed in the presence or absence of various concentrations of GTE and EGCG to test their effects on CYP2B6, CYP2C8, CYP2C19, CYP2D6 and CYP3A activities, respectively. Each metabolite was quantified using UPLC/ESI-MS, and the inhibition kinetics of GTE and EGCG on CYP enzymes was analyzed. In human liver microsomes, IC50 values of GTE were 5.9, 4.5, 48.7, 25.1 and 13.8 µg/mL, for CYP2B6, CYP2C8, CYP2C19, CYP2D6 and CYP3A, respectively. ECGC also inhibited these CYP isoforms with properties similar to those of GTE, and produced competitive inhibitions against CYP2B6 and CYP2C8, and noncompetitive inhibition against CYP3A. In human intestinal microsomes, IC50 values of GTE and EGCG for CYP3A were 18.4 µg/mL and 31.1 µM, respectively. EGCG moderately inhibited CYP3A activity in a noncompetitive manner. These results suggest that green tea catechins cause clinically relevant interactions with substrates for CYP2B6 and CYP2C8 in addition to CYP3A.