Fe-Pt-Mn thin films were deposited on quartz substrates and post annealed at a temperature range of 25-800 °C. The studied compositions include Fe_(50)Pt_(43)Mn_7, Fe_(50)Pt_(46)Mn_4, and Fe_(40)Pt_(56)Mn_4. X-ray diffraction data indicate that the Mn dissolving in the FePt lattice tends to stabilize the disordered fcc-FePt phase. The ordering transformation temperature was thus increased by about 150 °C. Besides, Mn also shows an effect of microstructure refining by 40%. L1_0 ordering can be enhanced by alloying a high concentration of 56 at% Pt. The higher degree of ordering enhances the coercivity value. δM analysis indicate that larger grain sizes and higher ordering significantly reduce the intergranular interactions of the Fe-Pt-Mn films.