NiFe2O4 nanoparticle series samples in the grain size range of 8—47nm were successfully synthesized by low-temperature solid-state reaction. The X-ray diffractometer (XRD)，HRPT neutron powder diffractometer，vibrating sample magnetometer (VSM) and superconducting quantum interference devices (SQUID) were used to characterize the structure，magnetic properties and surface anisotropy of nanoparticles. Data of XRD and neutron diffraction showed that the lattice constants of NiFe2O4 nanoparticles were lager than that of the bulk counterpart. Oxygen parameters suggested that lattice distortion were decreased with reduced particle sizes. In comparison with the bulk material，smaller magnetization，larger coercive force and anisotropy energy density for nanoparticles were observed. The critical sizes for transition from multidomain to single domain and for superparamagnetic transition were estimated to be 40nm and 16nm，respectively.