We investigate the characteristics of ZnO thin film transistor by thickness of ZnO active layers. The TFTs used in this experiment were all bottom-gate-type TFTs on Si(100) substrate. An Al of 300 nm thickness was used as a gate electrode, a 300 nm thick SiO2 as a gate insulator and a 300 nm thick Al as a source and drain electrode. The ZnO layers with the thickness of 30 nm to 300 nm were deposited by Radio-Frequency(RF) sputtering at room temperature. ZnO film was annealed by thermal oxidation furnace in N2 ambient at 300 ℃ for 2 hours after the deposition. The structural properties of ZnOfilms were investigated by an atomic force microscope(AFM) and a X-ray diffraction(XRD), and the electrical characteristics of ZnO-TFTs were measured using a semiconductor parameter analyzer(Agilent 4155C). As the active layer was thicker, the leakage current was lower and mobility was higher. Considering the leakage current and mobility, the 55 nm of ZnO active layer presented the best performance properties. On this occasion, the leakage current is 9.97 10-8 A, the channel mobility is 0.16 ㎠/Vs, and the threshold current is 12.7 V.