Yb 3þ /Er 3þ -codoped Gd 2 SiO 5 (Yb,Er:GSO) single crystal was grown by the Czochralski method. The upconversion properties of the crystal under 940 nm excitation were investigated. Intense emission bands centered at 521, 548, and 652 nm corresponding to the transitions 2 H 11/2 ! 4 I 15/2 , 4 S 3/2 ! 4 I 15/2 , and 4 F 9/2 ! 4 I 15/2 of Er 3þ , respectively, were observed at room temperature. The dependence of the intensity of the Er 3þ emission on the pump power and possible upconversion mechanism were discussed and evaluated. [9][10][11][12][13][14][15][16]. As the sensitizer, Yb 3þ ions perform an important role to reduce the lasing threshold and increase the pump efficiency, Er 3þ luminescence efficiency.Gadolinium oxyorthosilicate crystal Gd 2 SiO 5 (GSO) has been actively investigated recently as the excellent laser gain media because of their low symmetry, high quantum efficiency, and superior luminescence properties [17][18][19][20][21]. In this paper, we report the upconversion emissions of Yb,Er:GSO crystal under 940 nm excitation at room temperature and the upconversion mechanism from Yb 3þ to Er 3þ was discussed.