Experiments and analysis using the finite-element method were carried out to investigate the dynamic behavior of a thermally stressed disk with peripheral radial slits. (1) Natural frequencies tend to be unchanged by the thermal effect when the slit length is long. (2) Under the condition of constant slit length, the natural frequency of the two-radial-node mode which determines critical rotation speed, increases as the number of slits increases, and is constant when the number of slits is above three. (3) When double the number of radial nodes coincides with the number of slits or it's common multiple, this vibration separates into sin and cos modes. The natural frequency of the sin mode decreases rapidly as the slit length increases. (4) Suitable slit length for increasing the critical rotation speed is about 35% of the disk radius when the temperature difference between center and periphery of the disk is above 10°C.