The major non-P450 enzymes involved in the oxidative metabolism of drugs are: the flavin- containing monooxygenase (FMO), the monoamine oxidase (MAO), the aldehyde oxidase (AO), the xanthine oxidase (XO), the alcohol dehydrogenase (ADH) and the aldehyde dehydrogenase (ALDH). In recent years, the role of non-P450 enzymes in drug oxidative metabolism has garnered increasing attention. However, the contribution of non-P450 enzymes to the drug oxidative metabolism is possibly underestimated in many cases, as most metabolism studies in drug discovery and lead optimization are conducted using in vitro test systems related to P450 enzymes. In this article, these non-P450 enzymes in terms of catalyzed reaction types, common substrates, gene polymorphism and drug interaction are reviewed, and the in vitro models and factors for non-P450-mediated oxidative metabolism are summarized. Similar to P450 enzymes, non-P450 enzymes can directly catalyze the oxidation of drugs, yielding therapeutically active metabolites or toxic metabolites. These enzymes can also oxidize the toxic metabolites, generated from P450-catalyzed reaction, to nontoxic metabolites. In general, most non-P450 enzymes (such as FMO and MAO) appear to be much less inducible than P450 enzymes.