The microstructure and magnetic properties of (FePt)100-x(MgO)x films annealed at different temperature for different time were investigated. The (FePt)100-x(MgO)x films in situ deposited at 350 °C show (001) preferred orientation. The coercivity increases with the annealing temperature for all films. For FePt and (FePt)95 (MgO)5 films, the coercivity first increases with the increase of the annealing time from 10 to 30 s, but decreases dramatically with further increment of the annealing time. For the FePt–(MgO)10 film, the effect of annealing time on coercivity is not obvious. The domain size decreases with the increase of the MgO content and post-annealing can reduce the domain size effectively.