Gas-fired heating continues to be the main heating method in the steel processing industry. In the reheating furnace before hot rolling, the annealing atmosphere can vary significantly because of the fluctuation of the calorific value of gas. When the calorific value of gas is low, the residence time of billet in the furnace will be prolonged, which will affect the production rhythm. For the above reasons, the oxidation of steel billet in reheating furnace is difficult to predict and control. To achieve quantitative understanding and prediction of steel oxidation in a reheating furnace, an experiment was conducted to study the oxidation behavior of steel billet in simulated atmospheres at the temperature of a soaking zone of the heating furnace. The results of steel oxidation in high purity nitrogen with selected CO2 content at 1523 K (1250℃) are obtained. The result shows that oxidation kinetics curves of samples in different CO2 content are divided into a linear phase and a parabolic phase. In the linear phase, the time and weight gains of the linear phase decrease with CO2 content increase. In the parabolic stage, the parabolic rate constant has a logarithmic relationship with CO2 content. The oxide film on the surface of the sample is divided into a loose compact layer and a loose layer. The compact layer is located on the outside of oxide film consisting of Fe3O4 and its reduction product FeO, while the loose layer is between compact layer and metal matrix and is mainly composed of FeO as well as a few proeutectoid Fe3O4 during cooling. There are voids and cracks inside of the loose layer, and the higher the CO2 content is, the more voids there are. Condensation of vacancy was believed to be responsible for the formation of voids, while increased CO2 content can promote the process. By observing the surface morphology, it can be inferred that FeO in the compact layer might be generated by the reduction of Fe3O4.