The slab reheating process for iron rolling to the small diameter wire in Ratchasima Steel Productions Co., Ltd. factory (Nakhon Ratchasima, Thailand) use the Reheating Furnace Walking Hearth Type (RFWHT). The Reheating Furnace was installed in 1964. The problems in the present are increasing performance of temperature control and reducing production cost. The factors affecting the price of wire rod are electrical power and fuel consumption. Because of this reason, the system requires an optimal PI controller for control the temperature inside each zone of the furnace and the steel production manufacture select the time from 10:00 PM to 8:00 AM for steel production because it has low cost. In addition, the process will stop during the day time. This paper presents the mathematical model for temperature control of RFWHT in setting up process using system identification with Genetic Algorithm. The setting up process consist RFWHT, fuel flow control through servo valve and PI controller. The result of the research demonstrates to create structure of the mathematical model of this process and PI controller design for the temperature control. Furthermore, the temperature responses show that the PI controller can reduce fuel consumption rate of the system.