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An yttrium aluminum garnet (YAG) crystal fiber with a thulium-doped end tip was specially grown by means of the laser heated pedestal growth approach and designed to be incorporated in a fiber-optic temperature probe. The fluorescence decay characteristics of the crystal fiber, including the temperature dependence of both the fluorescence lifetime and intensity, were comprehensively investigated. Experimental results indicated that the crystal fiber showed a monotonic relationship between the fluorescence lifetime and temperature with an average lifetime sensitivity of 3 μs °C over a wide temperature range, taking measurement from room temperature to 1200°C. Good stability (up to 1400°C) was observed with high repeatability of the fluorescence lifetime during the annealing process carried out on the fiber over this temperature range. The fiber was found to be an excellent candidate material to be used as a fluorescence decay-based fiber thermometer probe and the results are presented on its performance.