In this work, we report a facile approach to significantly improve the electrical performances of a bottom-gated zinc oxide (ZnO) FET through In-situ annealing treatment of ZnO channel layer. We demonstrated ZnO FETs with extremely scaled channel thickness t ZnO of 3 nm, achieving low SS of 83 mV/decade and the highest µeff of 86 cm2/V•s. We offered insights into the sensitive role of interlayer dielectric passivation on oxide device stability, often neglected by prior work.