The expense of energy consumption is more than half of the operation cost in refinery. Therefore, how to increase the thermal efficiency of furnace is an important issue in reducing the operation cost. Some impurities in fuel oil, such as S, Ni… after burning will form deposits on the surface of convection tube. These deposits will decrease thermal efficiency. The purpose of this work is to develop a cleaner to efficiently remove the deposit from the surface of convention tube to improve its thermal efficiency. Design of experiments is used to study the effects of ingredients in the cleaner. It is also utilized to find the optimum formulation. Besides good results from laboratory test, field try also confirms the effective performance of the product.