Isolated and oriented hard magnetic FePtCu ternary alloy nanoparticles with the L10-type ordered structure have been directly fabricated by rf-magnetron sputtering onto NaCl substrate kept at 613K without any post-deposition annealing. Structure and morphology of those particles were studied by TEM. When the sputtering duration decreases, the particle size decreases and the particle areal number density increases. These nanoparticles have <100> orientation on the NaCl substrate. Alloy composition distribution from particle to particle examined by nano-beam elemental x-ray analysis was found to be quite small, which can be attributed to the homogeneous structure formed by co-deposition of Fe, Pt and Cu by sputtering. Thermal fluctuation of the magnetization becomes prominent in the specimen with the particle size smaller than about 10nm in diameter.