Abstract: In the PDF approach of turbulent reactive flows, the convection and reaction terms are closedbut the molecular diffusuion term is unclosed. The currently existing models for the moleculardiffusion are either empirical or introduced via external parameters, and it has been verifiedthat they are not valid for such physical processes as the diffusion process. In this paper,a stochastically mapping approximation approach is developed to model the effect of moleculardiffusion. A length scale is introduced by a two-point joint probability density function,to account for the evolutions of the conditional diffusion. Numerical simulation shows thatthe approach is able to predict the evolution of the variances, dissipation rates, conditionaldiffusion and PDFs in the diffusion process.