Self-aligned back-gated suspended body single-walled carbon nanotube field-effect transistors (SWCNT FETs) have been fabricated by a trench-assisted self-assembly precise positioning method based on dielectrophoresis. We report suspended body CNT FETs exhibiting high Ion/Ioff ratio up to 107 at a drain voltage of 0.1 V, an ultra low off state current, and a small subthreshold swing of 133 mV/decade at room temperature. The results are comparable with or even better than the state-of-the-art SWCNT FETs. Temperature dependency is also investigated. The self-assembled suspended body SWCNT FETs presented in this work have great potential for building resonant transistors for integrated sensing applications.