An isolation region is provided. The isolation region includes a first groove and an insulation layer filling the first groove. The first groove is embedded into a semiconductor substrate and includes a first sidewall, a bottom surface and a second sidewall that extends from the bottom surface and joins to the first sidewall. An angle between the first sidewall and a normal line of the semiconductor substrate is larger than a standard value. A method for forming an isolation region is further provided. The method includes: forming a first trench on a semiconductor substrate, wherein an angle between a sidewall of the first trench and a normal line of the semiconductor substrate is larger than a standard value; forming a mask on the sidewall to form a second trench on the semiconductor substrate by using the mask; and forming an insulation layer to fill the first and second trenches. A semiconductor device and a method for forming the same are still further provided. In the semiconductor device, a material of the semiconductor substrate is interposed between a second groove bearing a semiconductor layer for forming an S/D region and the first and second sidewalls. The present invention is beneficial to reduce leakage current.