The temperature dependence of the first-order magnetic anisotropy constant has been investigated for off-stoichiometric chemically ordered L10, Fe45Pt55 nanoparticle samples. A representative sample shows an anisotropy energy density of Ku=0.7×107 erg/cm3 and a thermal stability factor of KuV/kBT=77 at room temperature. With decreasing temperature, from room temperature to 5 K, a 50% increase of Ku, accompanied by a 25% increase of the saturation magnetization MS is observed. The temperature dependence of Ku is explained on the basis of the spin-Hamiltonian approach with an Fe effective spin. The estimated median “magnetic” grain size, obtained from time-dependent measurements, decreases by about 15% with decreasing temperature from 298 to 100 K. This magnetic grain size reduction is attributed to the decrease of the superparamagnetic fraction in the FePt nanoparticle sample. © 2003 American Institute of Physics.