A planar edge termination technique of trenched field limiting ring is investigated by using 2-dimensional numerical analysis and simulation. The better voltage blocking capability and reliability can be obtained by trenching the field-limiting ring site which would be implanted. The trench etch step makes the junction depth deeper so that junction curvature effect and surface breakdown are less happened. The numerical analyses reveal two facts that the trenched field limiting ring has smaller maximum electric field and the electric field peak is deeper from the substrate surface, hence silicon dioxide layer can be protected. Therefore the voltage blocking capability and reliability of the new structure can be improved. The simulated results for 1700V power devices by using TMA MEDICI show that the trenched field limiting ring can have smaller critical electric field and accomplish near 30% increase of breakdown voltage in comparison with the conventional structure. The proposed structure is more efficient to support voltage and more easy to gain the optimized design. Moreover, the fabrication of trenched field-limiting ring is relatively simple because the trench etch step uses the same mask of p+ field-ring implantation step. Extensive device simulations as well as qualitative analyses confirm these conclusions.