A study on the differential diffusion caused by the difference in the molecular diffusivities in turbulent H2/N2nonpremixed flames in coaxial air jet are investigated. The approach is to use the conventional conserved scalar method with the perturbation theory. The turbulence model used is κ - ϵ -gmodel. The combustion model assumes that instantaneous species concentrations and temperature are in a chemical equilibrium state. Then the fluctuations of these quantities are considered as clipped Gaussian probability density function. Computational results considering the differential diffusion forRe= 4200 are better than those which do not consider it. The effect appears great in fuel rich region which is consistent with the experimental result of Drake. The same computational procedure are done for high Reynolds number ofRe= 11000 to know the dependence of differential diffusion on the Reynolds number. As the Reynolds number increases, turbulent diffusion becomes dominant over molecular diffusion. So no sensible differences are found between the results by the perturbation method and by the conventional method.