Kinetic isotope effects (KIEs) in the electronic process of the H-atom abstraction from substrate ethane by a compound I model of cytochrome P450 are discussed at the B3LYP level of density functional theory. Our calculations demonstrate that the transition State for the H-atom abstraction involves a linear (Fe)O . .H . . .C array and that, a resultant radical species with a spin density of nearly one is bound to an iron-hydroxo complex, followed by recombination and release of product ethanol. Although the reacting system involves this carbon radical species in the course of the hydroxylation, in view of the energy profile of the reaction pathway it cannot be viewed as a stable reaction intermediate with a finite lifetime. The KIEs calculated with transition state theory are significantly dependent on temperature and substituents, falling in a range of 7-13 at 300 K.