Reliability issues associated with driving metal-oxide semiconductor thin film transistors (TFTs), which may arise from various sequential drain/gate pulse voltage stresses and/or certain environmental parameters, have not received much attention due to the competing desire to characterise the shift in the transistor characteristics caused by gate charging. In this paper, we report on the reliability of these devices under AC bias stress conditions because this is one of the major sources of failure. In our analysis, we investigate the effects of the driving frequency, pulse shape, strength of the applied electric field, and channel current, and the results are compared with those from a general reliability test in which the devices were subjected to negative/positive bias, temperature, and illumination stresses, which are known to cause the most stress to oxide semiconductor TFTs. We also report on the key factors that affect the sub-gap defect states, and suggest a possible origin of the current degradation observed with an AC drive. Circuit designers should apply a similar discovery and analysis method to ensure the reliable design of integrated circuits with oxide semiconductor devices, such as the gate driver circuits used in display devices.