We have produced self-assembled FePt nanoparticles by chemical synthesis and subsequent thermal annealing. The self-assembled samples were annealed in nitrogen or forming gas (95%Ar+5%H2) atmosphere for varying annealing time and temperatures. Thermal annealing above 500 °C resulted in phase transformation of the as-synthesized FePt from the chemically disordered fcc to chemically ordered fct structure which has a high uniaxial magneto-crystalline anisotropy. With increasing annealing temperature, the coercivity and the thermal stability factor KV/kT were found to increase due to improved chemical ordering, irrespective of the annealing atmosphere. For samples annealed at 580 °C for 30 min in forming gas, the maximum coercivity obtained was 6.95 kOe. The samples annealed in forming gas showed higher coercivity and KV/kT values than those annealed in nitrogen for the same annealing time and temperature. We have also measured the Curie temperatures of the FePt nanoparticle assemblies. The Curie temperatures were found to increase with increased annealing time for the samples annealed in nitrogen at 580 °C. © 2003 American Institute of Physics.