Abstract. Measurements of friction force of a pair of wheat grains against a flat plate sandwiched be-tween them were performed. Five levels of plate roughness and five levels of the grain moisture content were adopted. Two models were applied to describe the fric­tion force as a function of normal load: linear Coulomb's law of friction and two element non-linear equation developed from molecular-mechanical theory of friction. Both surface roughness and grain moisture content were found to influence model parameters. The Coulomb law of friction was found satisfactory for design calculations. Non-linear model appeared more accurate, particularly in the range of lower normal load. Moreover, the non-linear model parameters have been shown to be more closely related to the physical phenomena involved in the process of friction.