Intermetallic thin films have tunable magnetic properties. The magnetic phases of intermetallic thin films were tuned by changing the alloy composition of the intermetallic system. L10 Fe50Pt50 thin film has high magnetic anisotropy which makes them ideal candidates for the thin film recording media. Magnetic phases of Fe50Pt50 can be tuned by the addition of third element like Mn by forming Fe50-x Mnx Pt50 ternary alloy system. In this work magnetic phases of ordered Fe rich Fe50-xMnxPt50 and Mn rich Fe50-xMnxPt50 thin films of Fe50-x Mnx Pt50 alloy system is investigated. Fe rich Fe50-xMnxPt50 thin films are epitaxially grown on a- Al2O3 and MgO (100) substrates, while Mn rich Fe50-xMnxPt50 thin films are grown on MgO (100) substrates. The change in the magnetic properties in Fe rich Fe50-xMnxPt50 thin films due to presence of tetragonal phase and the prediction of a the presence of a new low temperature phase in the Mn rich Fe50-xMnxPt50 thin films is verified. These intermetallic films are produced in a Ultra High Vacuum sputtering system with Reflective High Energy Electron Diffraction and Auger electron spectroscopy. RHEED is used to verify epitaxy and Auger electron spectroscopy measures chemical composition.