**Abstract** : We explain shear thinning behavior observed in most concentrated non-Brownian suspen-sions by variable friction between particles. Considering the low magnitude of the forces experienced by the particles of suspensions under shear flow, it is first argued that rough particles come into solid contact through one or a few asperities. In such a mono-asperity elastic-plastic contact, the friction coefficient is not constant but decreases with normal load. Simulations based on Force Coupling Method are performed either with a constant friction coefficient or a load-dependent friction coefficient. Viscosity measurements are carried out on suspensions of polystyrene particles (40µm in diameter) dispersed in a Newtonian silicon oil. The simulations with a load-dependent friction coefficient qualitatively reproduce the observed shear thinning. Furthermore, the comparison between the simulations conducted either with constant friction coefficient or load-dependent friction coefficient provides a model for the shear-thinning viscosity. In this proposed model for the particles with load-dependent friction coefficient, the effective friction coefficient µ ef f is specified by the effective normal contact force which is simply proportional to the shear stress. As the shear stress increases, µ ef f decreases and the jamming volume fraction increases, leading to the reduction of the viscosity. A very good agreement between the model and the simulations is found and the overall agreement between the model and the experiments is satisfactory, especially regarding the variation of the jamming fraction with the shear stress. The closeness between the experiments and the results of the model enables us to evaluate the friction coefficient for each applied shear stress from the rheometric measurements.