Magnetic structures and magnetocrystalline anisotropy (MCA) of tetragonal phase Fe3Pt with an L1(2) atomic ordering in bulk and thin film are investigated by means of the first-principles full-potential linearized augmented plane-wave method. The results obtained predict that the tetragonal phase, in which the tetragonal distortion with (c/a) = 0.95 makes an asymmetry of magnetic properties along the c-axis (out-of-plane) and a-axis (in-plane), has an out-of-plane MCA. In addition, from the thin film calculations, the presence of surface is found to strongly enhance the out-of-plane MCA. (C) 2008 Elsevier B.V. All rights reserved.