Radiation stability of SiO2antireflective film coated SiN and SiC x‐ray mask membranes was studied. Although the transmission at 633 nm of the SiO2‐coated SiN was reduced by about 8% after synchrotron radiation (SR) exposure with an absorber dose of 100 MJ/cm3, the oscillation of the transmission due to interference was considerably suppressed even after the SR absorption. Furthermore, the optical transmission spectrum of the SiO2‐coated SiC was nearly equal to that of uncoated SiC after the SR absorption of 100 MJ/cm3. The pattern displacement induced by the SR absorption of 10 MJ/cm3for the SiO2‐coated SiN and for the SiO2‐coated SiC was σx=6 nm, σy=7 nm and σx=8 nm, σy=6 nm on the 25‐mm‐square area, respectively, the values of which were within the reproducibility of the displacement measurement. Electron spin resonance analysis indicated that no significant difference of spin density between the SiO2‐coated and uncoated SiN membranes was recognized before and after the SR absorption.