The present invention provides a method of forming a semiconductor device. First, a substrate having a first insulating layer formed thereon is provided. After forming an oxide semiconductor layer on the first insulating layer, two source/drain regions are formed on the oxide semiconductor layer. A bottom oxide layer is formed to entirely cover the source/drain regions, following by forming a high-k dielectric layer on the bottom oxide layer. Next, a thermal process is performed on the high-k dielectric layer, and a plasma treatment is performed on the high-k dielectric layer in the presence of a gas containing an oxygen element.