In this papar,a new atomic structure model of the Si{001}2×1 surface determined from band theory calcution of low-energy-electron diffraction are prensented.The new structure model involves a dimer band length ld = 2.387? and thee atomic layer distances d1 = 0.50±0.01?, d2= 0.96±0.01?, d3 = 1.17±0.01? and two asymmetric displacements of dimer atoms.