Nanoparticles of Co 1 – x Mg x Fe 2 O 4 with x equal to 0, 0.2, 0.4, 0.6, 0.8, and 1.0 have been synthesized. For all values of x , they are nanocrystals with a cobalt ferrite structure and an average linear size ( nm. Based on the analysis of the Mössbauer effect spectra, the Co 2+ ions were shown to occupy only octahedral positions at all values of x . The experimentally obtained dependence of the nanoparticles magnetization on x corresponds to the dependence calculated using the Mössbauer effect data, except for the sample with . The effective magnetic anisotropy constant estimated for 0 K from the analysis of the coercive force temperature dependences decreases from at to erg/cm 3 at and drops sharply to erg/cm 3 at .