Yb~(3+)-Er~(3+) co-doped K_2GdF_5 up-conversion phosphor was successfully synthesized by a solid-state reaction method. The phase purity and structure of the sample were characterized by powder X-ray diffraction. The sample emitted orange light at room temperature and its up-conversion spectra at different temperatures were recorded under the excitation of a 980 nm diode laser. The energy transfer from Yb~(3+) to Er~(3+) notably enhanced the up-conversion luminescence intensity. The possible up-conversion mechanisms and processes were proposed based on the power dependence of the luminescence intensities. The temperature-dependent up-conversion luminescence and temperature sensing performances of the sample were discussed according to the fluorescence intensity ratio of green emissions originating from ~2H_(11/2)/~4S_(3/2)→~4I_(15/2) transitions of Er~(3+) in the range from 307 K to 570 K under the excitation of 980 nm laser with power of 260 mW. The dependence of the fluorescence intensity ratio on temperature was fitted with an exponential function and the effective energy difference obtained was 690 cm~(–1), which further gave a relative temperature sensitivity of 1.1%/K at 307 K. The results suggested that the Yb~(3+)-Er~(3+) co-doped K_2GdF_5 sample is a promising candidate for optical temperature sensor. Yb ~ (3 +) - Er ~ (3+) co-doped K_2GdF_5 up-conversion phosphor was successfully synthesized by a solid-state reaction method. The phase purity and structure of the sample were characterized by powder X-ray diffraction. sample emitted orange light at room temperature and its up-conversion spectra at different temperatures were recorded under the excitation of a 980 nm diode laser. The energy transfer from Yb ~ (3+) to Er ~ (3+) notably enhanced the up- The possible up-conversion mechanisms and processes were proposed based on the power dependence of the luminescence intensities. The temperature-dependent up-conversion luminescence and temperature sensing performances of the sample were discussed according to the fluorescence intensity ratio of green emissions originating from ~ 2H_ (11/2) / ~ 4S_ (3/2) → ~ 4I_ (15/2) transitions of Er ~ (3+) in the range from 307 K to 570 K under the excitation of 980 nm laser with power of 260 mW. The dependence of the fluorescence in tensity ratio on temperature was fitted with an exponential function and the effective energy difference obtained was 690 cm -1, which further gave a relative temperature sensitivity of 1.1% / K at 307 K. The results suggested that the Yb ~ (3 +) - Er ~ (3+) co-doped K_2GdF_5 sample is a promising candidate for optical temperature sensor.