Because of the potential uses toward low-cost and flexible-substrate-based electronics, semiconducting organic materials have attracted much attention. In this contribution, structures and magnetic properties of Co-rubrene composite films on Si(1 0 0) have been studied by employing atomic force microscopy (AFM) and magneto-optic Kerr effect techniques. For composite films prepared by co-depositions of Co and rubrene on Si(1 0 0), the surface is smooth while a layered distribution of Co atoms is detected. For thick composite films, surfactant effects of rubrene molecules cause smooth surfaces and reduced interaction at the film/Si interface. For thin composite films, the formation of separated Co clusters in the films results in a larger coercive force due to the imperfection introduced by rough interface to impede the magnetization reversal. By increasing the rubrene concentration, more Co/rubrene interfaces are introduced in the composite films and the more rubrene served as a surfactant enhances the quality of the films. These information are valuable for future applications combining organic semiconductor and spintronics. (C) 2015 Elsevier B.V. All rights reserved.