Abstract: A method of cutting a tempered glass substrate, which is capable of cut a tempered glass substrate without mechanical breaking process and cooling process , is disclosed. The method of cutting a tempered glass substrate, includes, forming an initial crack at a cut-starting point of the tempered glass substrate; forming a heated line at the tempered glass substrate by irradiating laser beam from a point different from the cut-starting point with the initial crack to the cut-starting point with the initial crack through optical heater; and automatically propagating the intimal crack along the heated line to cut the tempered glass substrate.