Nickel ferrite nanoparticles were prepared by a hydrothermal method. The effects of synthetic conditions on particle sizes and magnetic properties were investigated by TEM, XRD, and VSM. The results show that the concentrations of metal ions were the key factors not only on the particle size of NiFe2O4 but also on their magnetic properties. The pH value influenced greatly the magnetic properties through the reacted impurities. Once the reactions were given high enough temperature and long enough time, the particles would reach their 'mature' size and relative stable magnetic properties due to more perfect structure and more relaxed structural stress, therefore they showed much lower Hc and higher Ms than that of "immatured" particles. During the growing period of the particles, however, the temperature and the time play predominant roles on the particle size and on the diminishment of impurity. In the growing period of these nanoparticles, the magnetic properties have been demonstrated to follow some size-dependent law. (C) 2009 Elsevier B.V. All rights reserved.