The invention provides a MOS transistor and a method for forming the MOS transistor. The MOS transistor includes a semiconductor substrate; a gate stack on the semiconductor substrate, and including a gate dielectric layer and a gate electrode on the semiconductor substrate in sequence; a source region and a drain region, respectively at sidewalls of the gate stack sidewalls of the gate stack and in the semiconductor; sacrificial metal spacers on sidewalls of the gate stack sidewalls of the gate stack, and having tensile stress or compressive stress. This invention scales down the equivalent oxide thickness, improves uniformity of device performance, raises carrier mobility and promotes device performance.