Abstract: The interactions between α-Al2O3 nanoparticles and metal ions or electrode surface during composite electrodeposition process were investigated by Zeta potential and electrochemical methods.Zeta potential measurements reveal that Al2O3 nanoparticles have a stronger tendency to adsorb Co2＋ on their surface than Ni2＋ in sulfamate electrolytes. Steady-state polarization and electrochemical impedance spectroscopy (EIS) indicate that the effect of Al2O3 nanoparticles on the Co-Ni electrochemical codeposition in different electrolytes ([Co2＋]/[Ni2＋]=1/5 and 5/1) is quite contrary. In nickel-rich electrolytes ([Co2＋]/[Ni2＋]=1/5),the Al2O3 particles have an inhibitory action on the reduction of Ni2＋, while in cobalt-rich electrolytes ([Co2＋]/[Ni2＋]=5/1), Al2O3 particles activate the reduction of Co2＋. That nanoparticles exhibit different influence on the reduction of Co2＋ and Ni2＋ were explained by considering the adsorption of metal ions on nanoparticle surface. Moreover, it can be seen from EIS that besides one capacitive loop at high frequency and one inductive loop at low frequency, the introduction of Al2O3 particles leads to exhibit a further inductive loop at low frequency. According to the simulated results, the electrochemical codeposition mechanism and simulated impedance parameters were analyzed and in od agreement with the experimental results.