New mechanism for oxygen activation in biological oxygenases has been proposed; in the present proposal the substrate is assumed to play an important role to activate the oxygen (or peroxide derivative) through donating electron to it, and several experimental facts to support the new idea have been provided. This idea suggests that formation of a high-valent iron-oxo species in cytochrome P-450 occurs only when the iron(III)-peroxide intermediate is activated through electronic interaction with substrate and also with the peripheral organic group, the latter of which also acts as an electron donor.