The influence of grain size on the magnetism in some ferreous magnetic nanoparticles was studied in experiment. It is found that the γ-Fe2O3 nanoparticles present the superparamagnetism. The coercivity and the remnant magnetization approach to zero. Moreover, the pure nanoparticles γ-Fe2O3 and Fe, under the normal temperature, the specific saturation magnetization decreases along with the grain size reduces. According to the basis magnetic domain structural theory, the grain size of the samples is all below the critical dimension ds.