This application discloses a Fin FET structure and a method for forming the same. In the Fin FET structure, there are lower stress spacers disposed over the lower portion of the fin's opposite sidewalls, asserting one stress type to suppress the carrier mobility; there are also upper stress spacers disposed over the upper portion of the fin's opposite sidewalls, asserting an opposite stress type to increase the carrier mobility. Therefore, the leakage current in the fin FET is reduced and the device performance is improved. In the method, the stress spacers are formed by depositing stress layers and etching back the stress layers, where stress types and magnitudes are controllable, resulting in a simple process.