The radiation force on a two-level atom in a sounding wave is calculated in thispaper,and the general expression of radiation force without spatial average is given.It isshown that the zero-order force corresponds to the stimulated radiation force, first-orderto the dipole force, while higher-order can be neglected. The magnitude and phase of thedipole force are given and the dependence of phase on atomic velocity is discussed, whichaffect the dynamics of stoms in a soanding wave. The explanation of phase dependence on atomic velocity is given by the photon redistribution model.