Effects of heat softening on the initiation of slide surface (shear banding) in clayey slopes during fast deformation were discussed. Controlling equations considering heat, pore pressure and mechanical movement were presented. By perturbation method, the instability condition of localized zone (i.e. criterion for initiation of shear banding) for thermal related soils, such as clayey slope, was obtained. It is shown that slide surface initiates once the thermal-softening effects overcome the strain-hardening effects whether it is adiabatic or not. Without strain hardening effects, strain rate hardening obviously plays a role in initiation of shear band. During initiating process, heat is trapped inside the shear band, which leads rapidly to a pore pressure increase and fast loss of strength. The localized shear strain is concentrated in a narrow zone with a width of several centimeters at most and increases fast. This zone forms the sliding surface. Temperature can increase more than 2°C, pore pressure can increase 160% in about 0.1s inside this zone. These changes cause the fast decrease in friction-coefficient by about 36% over the initial value. That is how shear band initiated and developed in clayey slopes.