Rare-earth-doped zinc oxide nanoparticles have been prepared with three different experimental methods, including precipitation method, alkali corrosion method and template method. The produced samples were analyzed on the upconversion optical properties. The structure analysis was performed on the samples by X-ray diffraction (XRD); the scanning electron microscope (SEM) was used to observe the powder's morphology and to estimate the particle size; the upconversion spectrum of sample was tested by fluorescence spectrometer and then the mechanism analysis was carried out. The results show that, the optical properties of nanoparticles are related to the size of particles. It has been proved that the smaller hollow spheres have better optical performance than that of the larger solid spheres. For hollow spheres, it is in favor of red upconversion when the size is smaller. On the contrary, it is in favor of green upconversion.