The quaternary-system Lu/In/Tm/Yb carbonat-recursors were synthesized by chemical precipitation route, and then were calcinated at 1100℃ to acquire a series of sphere-like [(Lu_0.5In_0.5)_0.999-x-Tm_0.001Yb_x]₂O₃ (x=0~0.05) solid solution-oxides with average particle size of about 110 nm. Under 980 nm laser excitation the oxide powder exhibits strong bluish emission at about 450∼510 nm and weak red emission at about 650~670 nm arising from ¹D₂→³H₆,³F₄ and ¹G₄→³F₄ transitions of Tm³⁺, respectively. Their upconversion mechanisms may be ascribed to the above two-phonon processes. The color of the emitted light on the 1931CIE color coordinates gradually move from green (0.31, 0.54) to blue (0.01, 0.19) colors with the increasing Yb³⁺ concentration. Yb³⁺ co-doping effectively enhances the luminescence intensity of Tm³⁺ and its optimum content is 2.5%. The fluorescence lifetimes of the oxide powder were measured to be about 0.84 for the 474 nm blue emission and 0.97 ms for the 654 nm red emission.