ZnO nanoparticles with different size were prepared by using sol-gel and precipitation method. With particle size of 10～50 nm, they were prepared by using precipitation method, and particle size and size distribution were controlled by milling process of an intermediate product Zn5(CO3)2(OH)6. With particle size smaller than 10 nm, they were synthesized by using sol-gel method, and the size was adjusted by changing the reaction conditions, such as reaction temperature, reaction time, etc. The ZnO nanoparticles with different size were characterized by using TEM, XRD, BET, UV-Vis. From UV-Vis spectra, the quantum effects of ZnO nanoparticles smaller than 10 nm were found, i.e., the absorption onset showed blueshifts with the decrease of size, and the ZnO nanoparticles bigger than 10 nm did not show quantum effects, the absorption edge was about 387 nm, corresponding to the bulk band gap for ZnO of 3.2 eV.