Submicron-sized CeO2:Er/Yb upconversion phosphor particles were synthesized by spray pyrolysis, and their luminescent properties were characterized by changing the concentration of Er and Yb. CeO2:Er/Yb showed an intense green and red emission due to the S3/2 or H11/2 → I15/2 and F9/2 → I15/2 transition of Er ions, respectively. In terms of the emission intensity, the optimal concentrations of Er and Yb were 1.0 % and 2.0%, respectively, and the concentration quenching was found to occur via the dipole-dipole interaction. Upconversion mechanism was discussed by using the dependency of emission intensities on pumping powers and considering the dominant depletion processes of intermediate energy levels for the red and green emission with changing the Er concentration. An energy transfer from Yb to Er in CeO2 host was mainly involved in ground-state absorption (GSA), and non-radiative relaxation from I11/2 to I13/2 of Er was accelerated by the Yb co-doping. As a result, the Yb co-doping led to greatly enhance the upconversion intensity with increasing ratios of the red to green emission. Finally, it is revealed that the upconversion emission is achieved by two photon processes in which the linear decay dominates the depletion of intermediate energy levels for green and red emissions for CeO2:Er/Yb phosphor.