We have fabricated an FePt nanoparticle assembly on a MgO(110) with a self-organized structure. Prior to FePt deposition, the MgO(110) substrate was annealed at 1273 K in air to develop self-organized grooves running along the  direction with average periodicity of 14.5 nm. Subsequently a very thin FePt layer with nominal thickness of 1.5 nm was sputtered onto the MgO substrate at 973 K, resulting in FePt L10 nanoparticles with 2–4 nm in diameter aligned along the grooves. Each FePt nanoparticle was a L10 single crystal with its c-axis tilting 45° or -45° from the substrate normal that corresponds to MgO or . The magnetic properties of the FePt nanoparticle assembly well reflected such c-axis distribution, and their switching behavior almost followed the Stoner-Wohlfarth model.