The present disclosure provides a semiconductor device including a metal gate structure and formation method thereof. The semiconductor device includes a substrate and a dielectric layer disposed on the substrate. The dielectric layer includes a trench. A diffusion barrier layer is disposed over a bottom surface and sidewall surfaces of the trench in the dielectric layer. The diffusion barrier layer includes at least a titanium-nitride stacked layer. The titanium-nitride stacked layer includes a TiNx layer disposed over the bottom surface and the sidewall surfaces of the trench, a TiN layer on the TiNx layer, and a TiNy layer on the TiN layer, x<1 and y>1. A metal gate is filled in the trench and disposed on the diffusion barrier layer.