MA956 and MA957 of oxide dispersion strengthened (ODS) steels are expected as future nuclear fusion material because of their superior mechanical properties and resistance to irradiation at high temperatures. However it was found that a cavity was formed upon heating to higher temperatures. After the materials were maintained for a long time at temperatures from 1 073 K to 1 473 K, the cavity formation and growth process were observed and analyzed. The shape of the cavity was spherical in MA956 and disk-like in MA957. The cavity volume fraction increased with the holding temperature and time. The reason for cavity formation and growth is that precipitates of Al2O3 and TiO2 become the nuclei in MA956 and MA957 respectively, and vacancies and Ar gas atoms are accumulated at the interface of the precipitates and matrix. For the growth rate of the cavity, good agreement between the experimental and theoretical values, considering the contribution of gas atoms, was obtained.