Abstract：The nonlinearity in loading process of frictional tests is discussed by using 3D spring-rider-flat model with moment considered. Due to the moment of driving force，the normal stress on the flat does not distribute uniformly，which induces nonuniform distribution of frictional resistance by assuming friction coefficient is constant. The whole loading process actually consists of two stages. (1) Before the shear stress reaches the resistance，the rider does neither slide nor deform and the force-displacement curve is linear. (2) After the shear stress，because of the moment，reaches the resistance firstly at the end where the driving force acts，the rider deforms before it slides as a whole. The deformation of the rider will lead to redistribution of the shear stress on the flat and the nonlinear curve of force-displacement should be considered. Since no damage happens to the flat，this mechanism can repeatedly produce such nonlinear phenomenon.
