ZnO thin films are prepared on glass substrate by sol-gel spin-coating method and the effect of preheating temperature on c-axis preferred orientation and optical properties of the films are investigated. The ZnO thin film that was preheated at 275 degrees C and post-heated at 650 degrees C using a sol with Zn concentration of 0.7 mol/l is highly oriented along the (002) plane. The transmittances of ZnO thin films preheated at 200-300 degrees C and post-heated at 650 degrees C are over 85% in visible range and exhibit absorption edges at about 368 nm. The optical band gap energy is evaluated to be 3.24 similar to 3.26 eV. The photoluminescence shows the ultraviolet emission at near band edge and broad green-yellow radiation at 490 similar to 620 nm. The effect of preheating temperature on structural and optical properties of ZnO thin films is discussed. (c) 2005 Elsevier B.V. All rights reserved.