Numerical analysis was conducted to solve the problems encountered in the thermal stress cleaving of borosilicate-glass plate of 50μm thick using CO₂ laser with the speed over 100mm/s. We investigated the influence of constraint and the effect of heat dissipation from the surface. It is concluded that the constraint of out-of-plane deformation does not influence the distribution of thermal stress intensity factor in the thickness and higher cleaving velocity may be attained with increase of heat dissipation.