A method to fabricate fiber Bragg grating (FBG) in an optical microfiber (OM) from a conventional photosensitive fiber is proposed in this letter. The cladding of a conventional photosensitive fiber is etched to 17 \mu m. The etched fiber is drawn to an OM 6 μm in diameter. The photosensitivity of the fabricated OM is effectively reserved. A FBG in the OM (MFBG) is successfully fabricated using a KrF excimer laser at a fluence of 400 mJ/cm2 through a phase mask with a pitch of 1 089.3 nm. The reflectivity of the FBG is approximately 10%, and the 3-dB spectrum bandwidth is 0.13 nm. The concentration of brine is measured by immersing the MFBG in the liquid, and the minimum detectable refractive index variation can reach 7.2 \times 10^{-5} at a refractive index value of 1.33.