Nanoparticles of MnFe2O4 fe"ite have been prepared by an aqueous phase co-precipitation method. The particles crystallize in spinel structure according to X-ray diffraction measurements (XRD) and transmission electron microscopy (TEM). The average crystallite size was found to change with duration of reaction which is in range of 15-25 nm as calculated from the Scherrer's formula. Magnetic measurements have been carried out by means of a vibrating sample magnetometer (VSM) showed that saturation magnetic moment Ms, coercive force He and Curie temperature Te of the samples increases with increasing. the particles size. The value of Te of the nanoparticle systems higher than the bulk material. Temperature Bocking has been determined by ZFC-FC measurements showed that super paramagnetic properties at the temperature higher than rooin temperature with Ta increased from 455K to 490K as the particle size increases. The value of the effective anisotropy Keff has been determined by correlations with temperature Ta. The contribution of surface anisotropy Ks and volume anisotropy Kv to the effective anisotropy of nanoparticles has been studied.