The effect of molecular weight distribution (MWD) on diffusion at symmetric polymer/polymer interfaces is investigated by rheological tools. A new model allowing the determination of a self-diffusion coefficient of polydisperse polymer systems is presented. The model is based on the double reptation theory and Doi and Edwards' molecular dynamics applied to A/A polymers brought into intimate contact in the molten state. The material parameters for the model are obtained from linear oscillatory shear experiments, in which the dynamic shear modulus is measured in parallel plate geometry under a small amplitude of deformation as a function of time and frequency for a sandwich-like assembly. The experiments were conducted on polystyrene (PS) blends with constant weight average molecular weight (M,,) but with variable number average molecular weights (M.). The measured self-diffusion coefficients showed that the presence of short molecules in the blend increases the mean value of the self-diffusion coefficient and the magnitude of such increase can be quantitatively evaluated by the proposed model.