ZnO:Yb³⁺,Tm³⁺,Ga³⁺ up-conversion nanoparticles (UCNPs) with sizes of between 20 and 65 nm are synthesized by the hydrothermal method. The size of the nanocrystals is affected by different Ga³⁺ concentrations. The variation in size has influence on the particles' visible blue and red up-conversion (UC) emission. In this paper, X-ray powder diffraction results show different amounts of gallium substituting for Zn in the ZnO lattice. When the concentration of Ga³⁺ is 3 mol%, the resulting UCNPs exhibit strong UC emission, longer fluorescence lifetime and narrower band gap. Due to their adjustable size and UC fluorescence properties, ZnO:Yb³⁺,Tm³⁺,Ga³⁺ nanocrystals have broad prospects for future applications in the biomedical field.