The mixing of thin granular layers transported on the surface of an oscillating trough is experimentally and numerically examined. The particle dispersion was experimentally quantified by an image processing system recording the growth of the mixing layer thickness of two differently colored but otherwise identical sand particle streams along the longitudinal position within the transporting channel. Granular flow and dispersion on the vibrating conveyor were studied numerically based on a three-dimensional discrete element code. Both experiments and simulations were used to derive quantities characterizing the transversal dispersion. The mixing was found to be directly proportional to the vertical acceleration of the conveyor and inversely proportional to the mass flow of the transported material. Keeping the above-mentioned parameters constant, the dispersion increases with increasing mean particle diameter. When performing the experiments with materials of different mean particle diameters and tuning the mass flow to achieve the same level of dimensionless bed height, the magnitude of the dispersion coefficient remains constant, as was also confirmed by the numerical simulation.