A MOS transistor structure with an in-situ doped source and/or drain and a method for forming the same are provided. The method comprises steps of: providing a substrate; forming a high Ge content layer on the substrate; forming a gate stack on the high Ge content layer and forming a side wall of one or more layers on both sides of the gate stack; etching the high Ge content layer to form a source region and/or a drain region; and forming a source and/or a drain in the source region and/or the drain region respectively by a low-temperature selective epitaxy, and introducing a doping gas during the low-temperature selective epitaxy to heavily dope the source and/or the drain and to in-situ activate a doping element.