Here, we investigate the effects of structural relaxa-tion (SR) on bias stability in solution-processed Ra-re-metal-free Zinc-Tin-Oxide (ZTO) thin-film transistors (TFTs). To demonstrate the effects of SR, the annealing time of ZTO semiconductor was controlled. As the an-nealing time increased, the negative bias stress stability deteriorated due to the SR effect, especially increase of oxygen vacancy in ZTO. In contrast, the localized states near conduction band in ZTO decreased owing to the re-duction of the free volume and the stabilization of the local atomic condition by additional thermal energy as the annealing time increased. As a result, positive bias stress stability was improved by SR. This study might contribute to the optimization of semiconductor activa-tion for minimizing the negative and positive bi-as-induced instability of newly emerged rare-metal-free oxide semiconductor TFTs.