A sub-10 µm long microfiber Bragg grating was nanostructured into a ~1 µm-diameter optical microfiber by focused ion beam (FIB) technology. The periodic structures were carved into the microfiber and the large refractive index contrast between glass and air allowed for the formation of strong gratings with only 20 periods. 3D simulation showed a good agreement with the experiment demonstration. This compact device can find applications in a variety of fields ranging from temperature and refractive index sensing to optical communications.