The effects of oxide on the microstructures and magnetic properties of L1(1) CoPt thin films were studied by adding SiO2 and MgO into CoPt thin films. The magnetic films were prepared by sputtering on MgO(111) substrates. SiO2 is found to segregate at the CoPt grain boundary, hindering the formation of L1(1) phase and degrading the perpendicular magnetic properties. Relatively, MgO nanoparticles precipitate in the CoPt matrix and stabilize the L1(1) phase. The high perpendicular magnetic anisotropy is well preserved even with a large MgO content in L1(1) CoPt-MgO composite film. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.