This paper discuss a stress control problem in a three-layered composite disk consisting of a transversely isotropic structural layer onto which piezoelectric sensor and actuator layers of crystal 6mm are perfectly bounded. It is considered that some electrodes are arranged concentrically on the upper surface of the actuator layer. When an unknown heating temperature distribution acts on the bottom surface of the disk, the electric potential distribution is assumed to be measured on the upper surface of the sensor layer. Then, the maximum thermal stress induced in the structural layer can be controlled by applying an appropriate voltage to each electrode. The voltages applied to electrodes are determined by optimization so that the maximum thermal stress in the structural layer is minimized subject to constrains on the stresses in the piezoelectric layers. Finally, numerical results are shown in tables.