Rare earth ions (Tm3+, Er3+, and Yb3+)-doped cubic Gd2O3 nanocrystal were prepared by a simple sol-gel method. Raman and FT-IR spectra were measured to evaluate the vibrational feature of the samples. Under 980 nm laser excitation, blue (488 nm), green (564 nm), and red (661 nm) upconversion has been recorded in Gd2O3:Tm+Yb and Gd2O3:Er (Gd2O3:Er+Yb), respectively. A great enhancement of red emission and diminishment of green emission of Er3+ in Gd2O3:Er+Yb have been observed. Laser power and doping concentration dependence of the upconverted emissions were studied to understand the upconversion mechanisms. Excited absorption and energy-transfer processes are discussed as the possible mechanisms for the visible emissions.