The short-time mutual diffusion at an interface of linear polystyrene/linear deuterated polystyrene with different molecular weights was examined by time-resolved neutron reflecdvity (TR-NR) measurements. The model scattering length density (b/V) profiles obtained by solving a partial differential equation for the diffusion process, using the segmental diffusion coefficients as the only fitting parameter were used to analyze the TR-NR data. In short-time diffusion, the asymmetric (b/V) profiles derived from a segmental relaxation model were able to model the data much better than those obtained by center-of-mass diffusion based on reptation model and by a simple error function. These analyses clearly indicate that even if the molecular weights of both components are larger than the critical molecular weight for entanglement, the initial interfacial broadening of bilayer films with different molecular weight proceeds with asymmetric mobility being proportional to N-1. Time dependence of the interfacial positions, u(t), was extracted from the (b/V) profiles. The exponent of a in u(t) similar to t(alpha) reflects the asymmetric mobility of the components. This analysis also gave the time dependence of mutual diffusion coefficients, D. The D value gradually decreases with increasing time and reaches a constant value.