Modulation Transfer Spectroscopy (MTS) has been proved to be a very effective method in laser stabilization.The asymmetric sidebands shift the line center of the MTS spectrum,and then re-duce the long term stability and reproducity of laser frequency.In this paper the formula for the line shape of MTS is derived based on the density matrix equations.The formula is applied for the calcu-lation of either the symmetric or asymmetric sidebands.Using this formula,The frequency shift due to the asymmetricity of the sidebands i s estimated.