Fiber Bragg gratings (FBG) were inscribed in single crystalline sapphire fibers by fs-laser irradiation. Due to the used multi-mode air clad fiber a sapphire-FBG spectra showa a wide asymmetric peak with a half width of 7 nm. Different mathematical peak functions were tested to determine a fiber Bragg wavelength. It was shown that the shift of the calcu-lated Bragg wavelengths in dependence on temperature is identical for the different peak functions. The determination of the fiber Bragg wavelength shift with a resolution of 10pm allows temperature measurements within an accuracy of ±1°C in the temperature range up to 1500°C. Sapphire FBG were used to measure the temperature distribution and thermal fluctuations within an inductive heated furnace in the range from 100°C to 1500°C.