The mechanism of composite additive in promoting reduction of copper slag was revealed. In this paper, the kinetic study of direct reduction of copper slag (CS) was studied by isothermal method, and the phase transformation and growth behaviors of metallic iron particles during reduction process were clarified by X-ray diffraction. Scanning Electron Microscopy, and Energy Dispersive X-ray Spectroscopy analyses. The results indicated that the apparent activation energy of reduction reactions of iron oxides within CS sample was decreased from 71.27 KJ/mol to 49.86 KJ/mol with the addition of 15% composite additive, and the mean particle size of metallic iron particles increased from 9.2 mu m to 35.2 mu m. The composite additive can promote the reduction of iron minerals and induce the adequate growth of metallic iron particles, which is beneficial for iron and copper recovery in magnetite separation process. (C) 2018 Published by Elsevier