YLiF4: Er3 + was synthesized by hydrothermal method. Concentration of Er3 + is changed from 0 to 5 %. The absorption of Er3+ in all samples from 200 to 1200 nm was measured at room temperature. The J-O parameters calculated from absorption spectrum areΩ 2 = 1.05 × 10- 20 cm2, Ω4 = 1.25 × 10- 20 cm2 andΩ6 = 1.35 × 10- 20 cm2. Infrared-to-visible upconversion emission of YLiF4: Er3 + was observed when excited by 980 nm. The results show that the Er3 + content is less than 1.5 %, excite-state absorption is the main mechanism of upconversion emission. When Er3 + content is larger than 1.5 %, both of the excite-state absorption and energy transfer lead to the upconversion luminescence. The upconversion intensity was enhanced with the increasing of Er3 + concentration. At room temperature, the lifetime of 2H11/2 and 4S3/2is 205μs while that of 4F9/2 is 188μs for sample Er-2. The transition rates and quantum efficiency were also calculated.The quantum efficiencies of 4S3/2 and 4F9/2 are 27.9% and 10.7%, respectively.