Sb-doped FePt nanoparticles with an average diameter of 8.5 nm were prepared by thermal decomposition of platinum acetylacetonate, antimony acetate, and iron pentacarbonyl. Upon annealing to ∼300 °C for 30 min, nanoparticles with XSb=0.14 and 0.23 show Hc≫500 mT, and L10 ordering parameter S values of ∼0.83–0.87. Transmission electron microscopy of the annealed assemblies shows no observable nanoparticle coalescence at 300 °C. Low-temperature coercivity measurements with a superconducting quantum interference device indicate the presence of particles that exhibit superparamagnetism, probably due to the large particle size distribution or inhomogeneity in Sb incorporation. Our results underscore the necessity to synthesize monodisperse FePt nanoparticles with controlled composition to maximize ferromagnetic behavior.