A semiconductor structure and a method for forming the same are provided. The semiconductor structure includes a substrate, an interfacial layer formed over the substrate, and an insertion layer formed over the interfacial layer. The semiconductor structure further includes a gate dielectric layer formed over the insertion layer and a gate structure formed over the gate dielectric layer. The insertion layer and the gate dielectric layer may be metal oxides where the insertion layer has an oxygen coordination number greater than the gate dielectric layer.