Abstract: The structural and binding properties of diatomic molecules CO, NO and O2 to P450 heme were investigatedin two different models (labeled as M1 and M2) using density functional method at the B3LYP/6-31G(d)level. The e?ects of the serine residue near diatomic molecules XO were considered in the model M2. Theresults show that the serine residue near the heme enforced the binding of XO to heme. Frequency analysisindicates that the stretching vibrational frequency decreased as CO, NO, and O2 complex with heme.