The dependence of room‐temperature oxidation of silicon catalyzed by Cu3Si on the silicide grain size has been investigated by transmission electron microscopy and x‐ray diffractometry. The thickness of the SiO2 layer was found to decrease with the average grain size of the starting Cu3Si layer. High‐resolution transmission electron microscopy revealed that oxidation is initiated at the grain boundaries. Oxide film as thick as 4.5 μm, compared to a previous record of about 2 μm, was grown at room temperature over a period of two weeks in (001) samples. The growth of thick oxide films was achieved by minimizing the grain size of Cu3Si through a reaction between Cu and an intermediate amorphous silicon layer at 200 °C.