Mo-Si-based superalloys are of interest as a new family of high-temperature materials to replace Ni-based alloys. By means of in situ environmental transmission electron microscope, the transient period of the early oxidation of Mo3Si was investigated at the nanometer scale. As a result of a competition between loss of volatile MoO3 and association of SiO2 molecular units into pillars, a nanometer-sized porous SiO2 grows at a very initial stage of the oxidation of Mo3Si.