Radiation mean free path (RMFP) is the function of radiation temperature, electron temperature and material density, and needs to apply Planck mean and Rosseland mean, respectively, in the regions of rarefaction wave and shock wave. The analytical formulae for the two kinds of RMFP have been given, by which the RMFP have been calculated for the interesting small atomic number Z. The calculated results show that the RMFP in non-local-thermal-equilibrium (non-LTE) and in LTE are obviously different and Planck RMFP is 30 times less than Rosseland one.