The removal of the oxide scales using hydrogen reduction will become a promising process compared to traditional pickling descaling due to its advantage for eliminating environmental pollution. However, decarburization occurs with the reduction of oxide scales and degrades the microstructures and properties of strip steel. To understand why decarburization occurs, a reduction experiment and a decarburization experiment on plain carbon steel were performed using a tubular furnace. The surface morphologies and microstructures of the cross-sections of the samples were observed by OM and SEM, and the weight losses of the samples were measured using a precision balance. The weight loss curves for hydrogen reduction and decarburization were plotted based on the experimental data. The decarburization mechanism is discussed in detail. The results show that the oxide scales reduce to pure ferrite and form a reduction layer on the surface of the strip steel after hydrogen reduction. A decarburized layer forms between the reduction layer and steel substrate. Decarburization is mainly caused by water vapor from the reduction of the oxide scales in a hydrogen atmosphere. Decarburization occurs simultaneously with the reduction of the oxide scales. The overall weight losses include deoxidation losses as well as decarburization losses. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.