Thickness dependent magnetism of the ultrathin Co films grown on SiO2 substrates was studied by surface magneto-optical Kerr effect in ultrahigh vacuum. The complicated thickness dependence of magnetization, coercive force and magnetic susceptibility was discussed in relation to surface morphology. Existence of 1 monolayer (ML) dead layer originated from partial oxidation at the Co/SiO2 interface. For the thinner films (similar to 2 ML), resistance to the magnetic reversal process was large because of larger surface roughness. For thicker films, reduction of the resistance was observed because of formation of continuous and defect less films. (c) 2005 Elsevier B.V. All rights reserved.