Yb3+ and Er3+ co-doped BaYF5: up-conversion nanocrystals are prepared by hydrothermal method. The sample is characterized by scanning electron microscopy and X-ray diffraction. Upon 980 nm laser diode excitation, the effect of hydrothermal synthesis temperatures on up-conversion luminescence properties is discussed. Results show that the sample obtained at 200 ℃ has larger green emission intensity than that of other samples, while the sample synthesized by hydrothermal method at 220 ℃ has the average grain size about 100 nm. According to the law of luminescence intensity versus excitation power, the 523 and 546 nm green emission, 649 and 667 nm red emission are all due to the double-photon process. The up-conversion mechanism and processes are also analyzed.