This paper reports on a novel optical fiber based sensor for hydrogen detection at high temperatures (>600°C). The optical sensor consists of a perovskite-type ceramic, SrCe 0.95 Tb 0.05 O 3−δ , as the sensing material, which is coated on the cladding layer of long period fiber gratings. Upon exposure to hydrogen, the reduction of Ce 4+ to Ce 3+ on the surface of SrCe 0.95 Tb 0.05 O 3−δ particle causes decrease in its refractive index. As a result, the resonance wavelength of long period fiber gratings shifts to a higher value. The rate of the shift is dependent on the hydrogen partial pressure and can be used as the sensing signal. The high temperature optical H 2 sensor exhibits high sensitivity for H 2 detection at a partial pressure of 0.005atm, and a short response time. The sensor is reversible as exposing the sensor to air oxidizes Ce 3+ to Ce 4+ .