Nanocomposite Cobalt-Carbon and Cobalt alloy-Carbon films show great application potential due to their unique structures and physical properties. By tuning the microstructures and composition, they can be used either as magnetic recording media or as soft magnetic materials. Recently, patterned magnetic nanostructure was fabricated using nonmagnetic Co-C thin film via phase change method. To better understand the phase-changing process and further improve the magnetic properties of such fabricated pattern, it is warranted to study its magnetic properties and microstructure as function of annealing temperatures and composition in detail. In this paper, a series of Co/C multilayer films were prepared and their magnetic properties and microstructures were studied detailedly.