A method for forming an insulating film on a grain-oriented silicon steel sheet by applying an insulating coating solution comprising100 parts by weight (calculated as the solid component) of a mixture of colloidal substances, composed of 50 to 98 weight % (calculated as the solid component SiO.sub.2) of colloidal silica having a partical diameter not larger than 50 nano m, and 2 to 50 weight % (calculated as the solid component) of at least one colloidal substance having a particle diameter randing from 80 to 3000 nano m, selected from the group consisting of oxides, carbides, nitrides, sulfides, borides, hydroxides, silicates, carbonates, borates, sulfates, nitrates and chlorides of Fe, Ca, Ba, Zn, Al, Ni, Sn Cu, Cr, Cd, Nd, Mn, Mo, Si, Ti, W, Bi, Sr, and V,130 to 250 parts by weight of at least one selected from the group consisting of phosphates of Al, Mg, Ca, and Zn, and10 to 40 parts by weight of at least one selected from the group consisting of chromic anhydride, chromate, and dichromate.The insulating coating produces good surface film lubricity and good heat resistance and excellent workability when fabricating into a wound core as well as excellent iron loss properties.