The precipitation process of nickel ferrite (NiFe2O4) from silicate glasses has been studied by electron spin resonance (ESR). The ESR linewidth &Dgr;H1/2and effectivegvalue were measured as functions of heat‐treatment temperature and time. Both the linewidth and effectivegvalue change drastically near the glass transition temperature and precipitation temperature. The results are interpreted on the basis of the development of ion ordering, i.e., magnetic‐order development, that occurs during the heat treatments.