We investigate thermally induced instability of thin film transistors with incorporating sputtered InZnO and InGaZnO as the channel layer at room temperature, using density of states extracted from temperature-dependent field-effect measurements method and falling rates of activation energy. The trends between density of states and stability under temperature stress are entirely consistent with each other in respect of the reduction of total traps with different channel layer. Moreover, the instability for InGaZnO based TFTs and InZnO based TFTs has been investigated under positive gate bias stress, and the device with InGaZnO channel layer shows a slighter positive threshold voltage shift than that of InZnO based device under positive bias stress. This result demonstrates that the stability of the device is closely related with the density of states.