A new ultrasonic method for determining temperature distribution of a thick plate being heated is presented. The principle of the method is based on temperature dependence of ultrasound velocity in heated materials. An inverse analysis method coupled with a finite difference calculation has been developed to determine one-dimensional temperature distribution in the heated plate. To demonstrate the feasibility of the developed method, a single side of a steel plate of 30 mm thickness is heated by contact with molten aluminum at 700°C and ultrasonic pulse-echo measurements are then performed for the steel during heating. The transit time of ultrasound of the heated steel is acquired and used to determine the transient variation of temperature distribution of the steel. The ultrasonically determined results almost agree with those measured using thermocouples installed in the steel.