In this work, ZnO thin films were prepared by sol–gel method and the dependence of structural and optical properties of these films on sol concentrations was deeply investigated. Unlike the previous studies, the ZnO thin films deposited in this study have approximately equal thickness, which excludes the influence of film thickness on the physical properties. The results show that low sol concentration is favorable for obtaining high c-axis oriented ZnO thin films with good crystalline quality. When the sol concentration is above 0.7 mol/L, the degree of c-axis orientation of ZnO thin films decreases and the optical quality is also degraded. Photoluminescence spectra indicate that the defect-related blue emission is increased with the enhancement of sol concentration. The mechanism of the blue emission is analyzed. The reason why high sol concentration is unfavorable for formation of high c-axis oriented ZnO thin films and obtaining high optical quality is also discussed.