We performed first-principles calculation for the body-centered cubic iron based on density-function theory, employing the pseudopotentionals and plane-wave method. We set the computational precision of the energy of one atom to 0.01 eV and make the spin-polarized total-energy calculation. The calculated results show that the 3p state should be treated as valence state when pressure is higher than 140 GPa, while the contribution to the total energy due to the dispersion of 3s state can be ignored for the whole range of earth's core condition.