A method for forming stressors in a semiconductor substrate is provided. The method includes providing a semiconductor substrate including a first device region and a second device region, forming shallow trench isolation (STI) regions with a high-shrinkage dielectric material in the first and the second device regions wherein the STI regions define a first active region in the first device region and a second active region in the second device region, forming an insulation mask over the STI region and the first active region in the first device region wherein the insulation mask does not extend over the second device region, and performing a stress-tuning treatment to the semiconductor substrate. The first active region and second active region have tensile stress and compressive stress respectively. An NMOS and a PMOS device are formed on the first and second active regions, respectively.