Martensitic transformation in a highly ordered Fe 3 Pt has been investigated by magnetization and X-ray diffraction measurements. We confirmed that a new type of face-centered tetragonal (FCT) martensite phase appears below 60K in Fe 3 Pt with a degree of order S=0.88. The tetragonality c/a gradually increases with decreasing temperature, and is approximately 1.005 at 10K. This is in contrast to the tetragonality c/a<1 generally observed in the FCT martensite with a degree of order less than 0.8. The spontaneous magnetization increases in association with the transformation.