The invention provides a method for improving gate structure line width roughness. The method comprises: providing a semiconductor substrate; forming a gate structure on the semiconductor substrate and forming a first isolating layer on the sidewall of the gate structure; forming a second isolating layer on the top of the gate structure, the top and the sidewall of the first isolating layer, and the semiconductor substrate; forming a sacrificial material layer on the second isolating layer; etching back the sacrificial material layer in order expose the second isolating layer on the top of the gate structure and the second isolating layer on the top of the first isolating layer; removing the second isolating layer on the top of the gate structure and the second isolating layer on the top of the first isolating layer by etching; removing the rest of the sacrificial material layer; removing the first isolating layer; performing annealing treatment in hydrogen atmosphere; and removing the second isolating layer. The method with annealing treatment in hydrogen atmosphere effectively controls the gate structure line width roughness within a certain range so as to improve the overall performance of a device.