We report on the fabrication of coplanar dual-gate ZnO thin-film transistors with 200-nm thickness SiNx for both top and bottom dielectrics. The ZnO film was deposited by RF magnetron sputtering on SiO2/Si substrates at 100. And the thickness of ZnO film is compared with 100-nm and 40-nm. This TFT has a channel width of 100-µm and channel length of 5-µm. The fabricated coplanar dual-gate ZnO TFTs of 40-nm-thickness exhibits a field effect mobility of about 0.29 cm2/V s, a subthreshold swing 420 mV/decade, an on-off ratio 2.7107, and a threshold voltage 0.9 V, which are greatly improved characteristics, compared with conventional bottom-gate ZnO TFTs.