The present study focused on the effect of nano-SiO2 on phosphate chemical conversion (PCC) coatings on 35CrMnSi steel prepared in PCC bath with addition of nano-SiO2 in the form of particles and sol. It was found that the microstructure and corrosion resistance of the coatings largely depended on the form and concentration of nano-SiO2. The proper addition of nano-Siv in PCC bath could significantly decrease crystal size and improve corrosion resistance. The coating formed in bath with addition of 100 ml/L SiO2 sol possessed a compact, uniform and fine-crystal microstructure as well as enhanced corrosion resistance. It was demonstrated that the alternative method of introducing SiO2 sol into PCC bath could avoid the agglomeration of nano-SiO2 drastically and make the coating have smooth crystal surfaces. (C) 2016 The Electrochemical Society. All rights reserved.