A transistor includes a semiconductor substrate, at least a gate structure, at least a first tensile stress layer, a second tensile stress layer, a source region, and a drain region. The gate structure is disposed within a first transistor region of the semiconductor substrate. The first tensile stress layer includes a curved portion encompassing the gate structure, at least an extension portion with a curved top surface located on the semiconductor substrate at sides of the gate structure, and a transition portion between the curved portion and the extension portion. The first tensile stress layer has a thickness gradually thinning from the curved portion and the extension portion toward the transition portion. The second tensile stress layer is disposed on the first tensile stress layer. And the source/drain regions are separately located in the semiconductor substrate on two sides of the gate structure.