Aristate spherical Ni nanoparticles with 5 nm-65 nm grain size range were synthesized via ultrasound reduction method using NiCl2.6H(2)O as raw materials, N2H4.H2O as reducing agent and PVP as surfactant. Phase and microstructure of as-synthesized Ni nanoparticles were characterized by X-ray diffraction (XRD) and high resolution transmission electron microscope (HRTEM). Magnetic properties were measured using vibrating sample magnetometer (VSM) at room temperature. Results indicated that the aristate spherical morphology of Ni nanoparticles could be ascribed to PVP surfactant effect on its nucleation and growth. Specific saturation magnetization M-s of the specimens increased from 26.3 A m(2)/kg (-1) to 48.0 A m(2)/kg(-1) as their grain size increased from 5 nm to 65 nm. Coercivity increased with their grain size. The maximum coercivity H-c reached up to 23.20 KA/m as their grain size increased to about 42 nm, indicating the experimental critical grain size for the aristate spherical Ni nanoparticles. The theory value for the ferromagnetic exchange length of Ni nanoparticles was about 49.1 nm, which was consistent with the experimental result. Their magnetic behavior maybe attributed to the effect of nanometer size on magnetic particles. (C) 2016 Elsevier Ltd. All rights reserved.