We report results of a systematic study on atomic and electronic structure of 55-atom and 147-atom Fe-Pt nanoparticles with different compositions using ab initio calculations. Our results on 55-atom nanoparticles suggest icosahedral structure and segregation of Pt on the surface to be favorable. Also, there is a tendency for Fe-Pt ordering on the surface and maximization of the unlike bonds similar to the bulk, while the core is made of pure Fe13, giving it a core-shell structure. Using these considerations, we designed a unique 147-atom icosahedral Fe75Pt72 nanoparticle in which a 55-atom core is made of Fe atoms and 72 Pt atoms are on the outer shell of the icosahedron. Twenty Fe atoms are on the centers of the Pt hexagons on the faces of the icosahedron. This nanoparticle has 260 μB magnetic moments, and the local magnetic moments on Fe and Pt atoms are higher compared with the values in bulk FePt. The free Fe55 cluster has a distorted icosahedral structure to be most favorable, with large magnetic moments (∼3 μB/atom), which we also find in the core of the nanoalloy particle, but the surface Fe atoms have higher magnetic moments of about 3.4 μB/atom. All the Pt atoms are on the surface with Fe-Pt ordering in a nearly symmetric icosahedral structure, and the heat of formation is the highest, which makes it an optimal nanoparticle. The overall composition of this nanoparticle is FePt-like, but it decomposes to Fe core and Fe-Pt shell. © 2015 American Chemical Society.