Abstract: As the foundation of the microcosmic study on shear strength of structural planes, the contact friction model of particles has never been built rationally because of the complex plastic deformation under high stress. Although the character of asperity micro-displacements caused by a tangential load under perfect elastic or fully plastic contact states was studied clearly by Fujimoto in 2000, it cannot be used to explain the friction mechanism of geotechnical particles with plastic hardening deformations. Here taken the Fujimoto model as the foundation, and based on the plastic hardening contact theory ever proposed by the author, the contact friction mechanism of plastic hardening particles is researched systematically so as to illustrate the relation between friction and tangential displacements with different contact states. At last, an example is given to verify the application of the theoretical method proposed. The result shows that the friction of micro-asperities is the sum of different kinds of friction on different contact areas with different contact states, and the start of sliding is a process that a micro-slip area increases while stick zone disappears.