Pure Ni nanoparticles ranging in size from 24 to 200nm are prepared via thermal decomposition of nickel acetylacetonate in oleylamine.The as-prepared Ni particles change from spherical to dendritic or starlike with increasing precursor concentration.The particles are stable because the organic coating occurs in situ.Magnetic measurement reveals that all the Ni nanoparticles are ferromagnetic and show ferromagnetic-paramagnetic transitions at their Curie points.The saturation magnetization Ms is sizedependent,with a maximum value of 52.01 and 82.31 emu/g at room temperature and 5 K,respectively.The coercivity decreases at first and then increases with increasing particle size,which is attributed to the competition between size effect and shape anisotropy.The Curie temperature Tc is 593,612,622,626 and 627 K for the 24,50,96,165 and 200 nm Ni nanoparticles,respectively.A theoretical model is proposed to explain the size-dependence of Ni nanoparticle Curie temperature.