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A numerical study in methane/hydrogen diffusion flames has been conducted to clarify the preferential diffusion effects of H2 and H with detailed chemistry. The composition of fuel is systematically changed from pure methane to pure hydrogen through the molar addition of H2 to methane. A comparison was made by employing three species diffusion models, i.e., mixture-averaged species diffusion and the suppression of the diffusivities of H and H2. The behavior of maximum flame temperatures with the three species diffusion models is not explained by the scalar dissipation rate but the nature of chemical kinetics such as the behaviors of chain carrier radicals of H, O, and OH. It is found that the preferential diffusion of H radical into the reaction zone curbs the populations of the chain carrier radicals and then flame temperature while that of H2 into the reaction zone produces the reduction of the scalar dissipation rate and the population of chain carrier radicals and these force the flame temperature to decrease. These preferential diffusion effects of H2 and H are also compared among NO emission behaviors through the three species diffusion models. Under all flame conditions, Fenimore NO is much more remarkable compared to thermal NO. It is also seen that the preferential diffusion of H radical into the reaction zone suppresses the thermal and Fenimore NO while that of H2 into the reaction zone increases them. To facilitate the details of those NO behaviors through preferential diffusion effects of H2 and H, importantly contributing reaction steps to the production and destruction of Fenimore NO are addressed.