Numerical study is conducted to grasp preferential diffusion effects on flame characteristics in H2/CO syn-gas diffusion flames diluted with CO2. The models of Sun et al. and David et al., which have been well known to be best-fitted for H2/CO synthetic mixture flames, are evaluated for H2/CO synthetic mixture flames diluted with CO2. Comparison of flame structures with mixture-averaged species diffusion and suppression of the diffusivities of H2 and H was made. The behaviors of maximum flame temperatures with those species diffusion models are not explained by scalar dissipation rate but by the nature of chemical kinetics. Importantly-contributing reaction steps to heat release rate are also compared for the three species diffusion models in H2/CO/CO2 flames with and without CO2 dilution.