A new method to determine total heat exchange factor was proposed for the prediction of slab tem-perature in reheating furnace. This method employed the square of the difference between measured and predicted temperature as an objective function. The slab temperature and surface heat flux were calculated under the condition minimizing the objective function. The real slab temperature in a walking beam type reheating furnace with 36m of its effective length was directly measured by using a heat resistive data logger system. The present method was validated by showing that the pre-dicted slab temperature was in a good agreement with the measured one.