The invention provides a method for manufacturing a transistor which includes: providing a substrate having a plurality of transistors formed thereon, wherein each transistor includes a gate; forming a stressed layer and a first oxide layer on the transistors and on the substrate successively; forming a sacrificial layer on the first oxide layer; patterning the sacrificial layer to remove a part of the sacrificial layer which covers on the gates of the transistors; forming a second oxide layer on the residual sacrificial layer and on a part of the first oxide layer which is exposed after the part of the sacrificial layer is removed; performing a first planarization process to remove a part of the second oxide layer located on the gates of the transistors; performing a second planarization process to remove the residual second oxide layer; and performing a third planarization process to remove the stressed layer.