The electro-reduction of magnetite is important for many different industries. For example, it relates to the extraction of iron from iron ore and the charging of iron-based batteries. The sluggish electro-reduction reaction of magnetite affects the energy efficiency of iron extraction or the charge rate of iron-based batteries. However, what factors affecting the reduction rate is not well understood. In this study, electrochemical analysis, XRD, SEM, EDS mapping and XPS techniques are adopted to analyze the reaction, and it is proved that two factors determine the electro-reduction rate of magnetite electrode: O-2(-) diffusion inside the particles determines the electro-reduction rate of a single particle, and the electrical resistivity of electrode determines how many magnetite particles are involved in electro-reduction simultaneously. Contact resistance between particles is the dominant part of the electrical resistance of the magnetite electrode. The function mechanism of sulfide in the reaction is also revealed: a sulfide additive can react with magnetite to form amorphous or crystal FeS at different potentials. These compounds, especially crystal FeS, coat onto the surface of magnetite particles, and bond them together. The coating and bonding can decrease contact resistance significantly, thus enhancing the electro-reduction rate of magnetite to one order of magnitude. (c) 2017 Elsevier Ltd. All rights reserved.