The work presents the assumptions of a complex numerical thermo-kinetic model for Friction Stir Processing. The model makes it possible to calculate the under-tool temperature of the surface of a material being modified as well as to determine lines corresponding to the shifting of a material modified during FSP. The calculation results indicate the influence of a tool rate of rotation and of a tool travel rate on the concentration of lines and on the surface temperature. The article also reveals that a material being modified is composed of two zones in which the material is shifted, i.e. the primary and secondary zone.