EGFR inhibitors, even with therapeutics superiorities in anticancer, can cause idiosyncratic pulmonary and hepatic toxicities that are associated with the reactive electrophile bioactivated by Cytochrome P450s (P450s). Until now, neither has the electrophilic intermediate been caught experimentally, nor has the subtle mechanism been declared. Herein, the underlying mechanism of bioactivation mediated by P450s was explored by DFT calculations for a case of EGFR inhibitor, Erlotinib. Based on the calculation and analysis, we suggest that with other metabolites, reactive electrophiles of Erlotinib: epoxide and quinine-imine, can be generated by several steps along the oxidative reaction pathway. The generation of epoxide needs two steps: (1) the addition of Erlotinib to Compound I (Cpd I) and (2) the rearrangement of protons. Whereas, quinine-imine needs a further oxidation step (3) via which quinone is generated and ultimately turns into quinine-imine. Although both reactive electrophiles can be produced for either face-on or side-on pose of Erlotinib, the analysis of energy barriers indicates that the side-on path is preferred in solvent environment. In the rate-determining step, e.g. the addition of Erlotinib to the porphyrin, the reaction barrier for side-on conformation is decreased in aqueous and protein environment compared with gas phase, whereas, the barrier for face-on pose is increased in solvent environment. The simulated mechanism is in good agreement with the speculation in previous experiment. The understanding of the subtle mechanism of bioactivation of Erlotinib will provide theoretical support for toxicological mechanism of EGFR inhibitors.