FePt nanoparticles were prepared by multi-step reduction method. Effects of annealing temperatures on the magnetic properties of the nanoparticles were investigated. The results of XRD and TEM confirm that the as-synthesized FePt nanoparticles were fcc structure and monodisperse, with spherical shape and around 5.0 nm particle size. The results of DSC and VSM show that FePt nanoparticles would change into ordered fct structure after annealing at 500 °C for 30 min from disordered fcc structure. With the temperature increasing, the coercivity was increased, which was 240 kA/m at 600 °C. However, it changed slightly in the high-temperature region (550 °C and above it), which was attributed to the agglomeration of nanoparticles during the annealing treatment.