The electrochemical removal of diclofenac (DCF) in aqueous solution was investigated by three-dimensional electrofenton (3DEF) process using Ti/TiO2-RuO2 anode electrode in the presence of composites of multi-walled carbon nanotubes (MWCNTs) and magnetite (Fe3O4) nanoparticle. The response surface methodology (RSM) coupled with central composite design (CCD) was used to evaluate the effects of different variables on the removal of DCF. Based on the experimental results, an empirical relationship between response and independent variables was obtained and expressed by a quadratic polynomial equation. Maximum DCF removal was 98.52% in optimum conditions at initial pH of 5.56, DCF concentration of 6.71 mg/L, the current density of 19.74 mA/cm(2), MWCNTs-Fe3O4 concentration of 58.33 mg/L, and electrolysis time of 82.24 min. The chemical oxygen demand (COD) removal efficiency and H2O2 production in the 3DEF system were much higher than other electrocatalytic processes. This is related to the presence of MWCNTs-Fe3O4 as a particle electrode that can activate the molecular oxygen for higher production of hydrogen peroxide (H2O2) and hydroxyl radical (center dot OH). The MWCNTs-Fe3O4 nanocomposite indicated a high degree of stability and reusability. Additionally, 1-(2,6-dichlorophenyl) indolin-2-one, 2,6- dicholoaniline, 2,4- dichlorophenol, and 1,2- benzene dicarboxylic acid were identified as main oxidation products after 60 min reaction. Based on the reaction intermediate identified, a possible pathway for the electrochemical oxidation of DCF by the 3DEF process was proposed. Eventually, the 3DEF system with MWCNTs-Fe3O4 particle electrodes can be considered as a viable alternative for DCF removal from aqueous solution. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.