Transparent thin film transistors (TTFT) were fabricated on N+ Si wafers. SiO2, Si3N4/SiO2 and Al2O3/SiO2 grown on the wafers were used as gate insulators. The rf magnetron sputtered zinc tin oxide (ZTO) films were adopted as active layers. N+Si wafers were wet-oxidized to grow SiO2. Si3N4 and Al2O3 films were deposited on the SiO2 by plasma enhanced chemical vapor deposition (PECVD) and atomic layer deposition (ALD), respectively. The mobility, Ion/Ioff and subthreshold swing (SS) were obtained from the transfer characteristics of TTFTs. The properties of gate insulators were analyzed by comparing the characteristics of TTFTs. The property variation of the ZTO TTFTs with time were observed.