Er3+/Yb3+:Gd3Sc2Ga3O12 and Er3+:Gd3Sc2Ga3O12 crystals have been grown by Czochralski method. The absorption spectra from 320 to 1700nm and the up-conversion fluorescence spectra from 500 to 750nm of them were investigated at room temperature, respectively. The possible up-conversion luminescence mechanisms in Er3+/Yb3+-codoped crystals and their influences on the 2.8μm laser emission were discussed. Experimental results show that the absorption of Er3+:Gd3Sc2Ga3O12 around 966nm and its bandwidth are remarkably increased by Yb3+ sensitizing. Under 940nm excitation, the up-conversion luminescence intensity of Er3+/Yb3+:Gd3Sc2Ga3O12 is much stronger than that of Er3+:Gd3Sc2Ga3O12. It demonstrates that there exist effective energy transfer processes between Yb3+ and Er3+, and the dominant mechanism of up-conversion may be the energy transfer processes of Yb3+-Er3+ and Er3+-Er3+.