The influence of grain size on the lattice constant in some nanocrystallites was studied by in experiment. It is found that the lattice constant of NaCl, CsCl structure ionic crystallites decreases with the reduction of the grain size. The lattice constant of ultrafine iron and nickel particles increases with the reduction of the grain size. As to the experiment result of g-Fe2O3 nanocrystallites, the lattice constant increases with the decreasing of the grain size. It is opposite to the theoretical result of ionic crystallites, which reason is that the combination of most of atoms in g-Fe2O3 is by the covalent bond other than electrovalent bond.