Methods for manufacturing a semiconductor device having a dual gate dielectric layer may include providing a substrate including first and second regions, forming a first gate dielectric layer having a first thickness on the substrate, forming an interlayer insulating layer including first and second trenches exposing the first gate dielectric layer in the first and second regions, forming a sacrificial layer on the interlayer insulating layer and bottoms of the first and second trenches, forming a sacrificial pattern exposing the first gate dielectric layer of the bottom of the first trench, removing the first gate dielectric layer of the bottom of the first trench, forming a second gate dielectric layer having a second thickness on the bottom of the first trench, removing the sacrificial pattern, and forming a gate electrode on each of the first and second gate dielectric layers.