A work function metal gate device includes a gate, a drift region, a source, a drain and a first isolation structure. The gate includes a convex stair-shaped work function metal stack or a concave stair-shaped work function metal stack disposed on a substrate. The drift region is disposed in the substrate below a part of the gate. The source is located in the substrate and the drain is located in the drift region beside the gate. The first isolation structure is disposed in the drift region between the gate and the drain.