The normalized efficiency T of thermal power dispersion and normalized efficiency C of chromatic power dispersion of optical materials are introduced. Based on the mutual compensation of thermal dispersion and chromatic dispersion, the equations are formulated for athermalization of optical system. The suitable materials of athermal optical system can be selected using the T-C chart. The defocus, lateral aberration and modulation transfer function(MTF) of two athermalized lenses with the same parameters are designed using the method, are given as examples.