In this work, we study the impact of gate dielectrics on the characteristics of bottom-gated tin-oxide thin-film transistors annealed in an oxygen ambience at 300 °C for various periods. SiO2, HfO2, and Al2O3 are employed as the gate dielectric in the test devices. The results show that the devices will start exhibiting p-type conduction behavior as the annealing reaches a specific time period which is closely related to the underlying gate dielectric, and the device characteristics can be improved as the annealing proceeds further. Nonetheless, a prolonged annealing may cause degradation of the devices. High hole mobility (3.33 cm2V%1 s%1), low threshold voltage (1.95 V), and excellent Ion/Ioff ratio (>104) are achieved on SnO TFTs with a SiO2 gate dielectric after an annealing of 30 min.