A long-period grating (LPG) based compact optical fiber sensor working in reflection mode is demonstrated. A technique to make a mirror on the cladding region of a fiber end-face to reflect only the cladding modes was realized by growing a polymeric microtip on the core region of the fiber end-face, by photopolymerization, followed by coating the fiber end-face with an aluminum film. Using the cladding-mode-selective fiber end-face mirror, the transmission spectrum of the LPG was "inverted" and reflected. Preliminary results of using the sensor to measure the refractive index of glycerol/water solutions were successfully demonstrated.