Semiconductor devices with dual-metal gate structures and fabrication methods thereof. A semiconductor substrate with a first doped region and a second doped region separated by an insulation layer is provided. A first metal gate stack is formed on the first doped region, and a second metal gate stack is formed on the second doped region. A sealing layer is disposed on sidewalls of the first gate stack and the second gate stack. The first metal gate stack comprises an interfacial layer, a high-k dielectric layer on the interfacial layer, a first metal layer on the high-k dielectric layer, a metal insertion layer on the first metal layer, a second metal layer on the metal insertion layer, and a polysilicon layer on the second metal layer. The second metal gate stack comprises an interfacial layer, a high-k dielectric layer on the interfacial layer, a second metal layer on the high-k dielectric layer, and a polysilicon layer on the second metal layer.