We investigated the MgO layer thickness dependences of the structure and magnetic properties of L1 0-FePt/MgO/GaAs structures. To examine how the crystallinity and growth morphology of the MgO layer affect the L1 0-FePt layer, two kinds of preparation method were employed for MgO deposition: electron beam (EB) evaporation and sputter deposition. The MgO layer deposited by EB evaporation included a large strain because of the cube-on-cube epitaxial relationship despite a large lattice mismatch between MgO and GaAs. For the MgO layer prepared by sputtering, on the other hand, an amorphous MgO layer was initially grown on the GaAs substrate. Subsequently, a crystalline MgO layer was grown in the (001) direction. In the case of the EB-deposited MgO, as the MgO layer thickness increased, the degree of chemical order of the L1 0-FePt layer increased from 55 to 81% owing to the improvement of the crystallinity of the MgO layer. The improvement of chemical order also led to the increase in the remanent magnetization of L1 0-FePt from 84 to 98%.