Ignition temperature of 2Al/Fe₂O₃ thermite was studied by analysis and experiments. Failure temperature of aluminum particle oxide film was analyzed according to the weight,resistance and appearance of aluminum powder undergoing different temperatures. Critical ignition temperatures of thermite spheres with different radii were calculated by Frank-Kamenetskii theory and measured by a high-temperature muffle furnace. Results show that the protective film fails almost completely at the melting point of aluminum,and ignition occurs at temperature of about 1600 K, and the independent of the volume of thermite sphere and ignition temperature is not correctly calculated by the Frank-Kamenetskii theory of homogeneous combustible system.