The present disclosure provides HKMG transistor structures and fabrication methods thereof. An exemplary method includes providing a base substrate having a first region and a second region; forming a dielectric layer having a first opening in the first region and a second opening in the second region over; forming a gate dielectric layer on a side surface of the first opening and a portion of the base substrate in the first opening and on a side surface of the second opening and a portion of the base substrate in the second opening; filling a sacrificial layer in the first opening; forming a second work function layer in the second opening and a second gate electrode layer on the second work function layer; removing the sacrificial layer; and forming a first work function layer in the first opening and a first gate electrode layer on the first work function layer.