ZnO nanoparticles with three different diameters were synthesized using a chemical refluxing route, which were then spin coated on indium tin oxide coated glass substrates. Three ZnO nanoparticle thin films denoted as samples a, b and c was obtained. The results of FESEM images showed that the crystal grains in all thin films were approximately particle shape with an average size of 5 nm, 25 nm and 40 nm, respectively, and all thin films were smooth. The XRD results indicated that ZnO nanoparticles displayed a hexagonal wurtzite structure. Samples a and b revealed weak absorption in the visible light region and strong absorption in the ultraviolet region, while the sample c both showed strong absorption in the visible and ultraviolet region, which was contributed to the ZnO nanoparticles with large diameter in the sample. Photoluminescence spectra at room temperature displayed that both a near band edge (NBE) ultraviolet photoluminescence peak and a blue peak were observed in sample a, both samples b and c exhibited a defect related deep level emission band. This was due to a large number of intrinsic defects in three thin films.