Measurements of friction force of a pair of wheat grains against a fiat plate sandwiched between 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.