The present disclosure provides a semiconductor structure includes a gate structure disposed over a substrate, wherein the gate structure includes a high-k dielectric layer and a work function structure. The high-k dielectric layer includes a base portion and a side portion, the side portion is extended from an end of the base portion, the side portion is substantially orthogonal to the base portion. The work function structure includes a first metal disposed over the high-k dielectric layer and a second metal disposed over the first metal and including a bottom portion and a sidewall portion extended from an end of the bottom portion, wherein the first metal includes different materials from the second metal, and a length of an interface between the sidewall portion and the bottom portion to a length of the bottom portion within the high-k dielectric layer is in a predetermined ratio.