In this paper, we performed first-principles calculations for iron with different crystal structures based on density-function theory, employing the pseudopotentional and plane-wave method. We set the computational precision of energy to 0.01 eV/atom and made total-energy calculations. The calculated results show that: 1) The bcc iron undergoes a pressure-induced phase transform to hcp structure at about 15 GPa, which is in good-agreement with the experimental values; 2) The magnetic moment decreases with increasing pressure, which demonstrates that the high pressures will destroy the magnetism of iron; 3) The ratio of lattice parameters of hcp structure, c/a, will slowly increase with the pressure increasing. When set to a constant about 1.59, the c/a value can give the energy computation precision of 0.01 eV/atom under the pressures in the earth core.