The comparative study of atomic and electronic structure of hem complexes of hemoglobin with molecular oxygen and nitric oxide has been performed by semiempirical quantum chemical PM3 method. It has been shown that the length of chemical bonding in oxygen molecule coordinated with hem increases by 0.046 A and the length of chemical bonding in nitrogen oxide coordinated with hem increases by 0.064 A in comparison with pure substances. This fact indicates that chemical bonding between nitric oxide and Fe atom of hem is stronger that one with oxygen molecule. Analysis of charge of the molecules indicates that NO bounded with Fe by covalent chemical bonding and oxygen molecule bounded with Fe by weak dipole interaction. Atomic orbitals of ligand atoms in oxygen complex play small part in high occupied (HOMO) and low vacant (LVMO) molecular orbitals in comparison with HOMO and LVMO of complex with NO. In the last one unpaired electron of NO molecule moves from ligand to d-orbitals of Fe atom and creates d7-configuration. Molecular dynamics simulation under physiological temperature (310 K) indicates visible difference in atomic and electronic structure of the complexes in comparison with ones under low (from 77 up to 0 K) temperatures.