This paper addresses the on-resistance (Ron) extraction of the DMOS based driver in Power IC designs. The proposed method can extract Ron of a driver from its layout data for the arbitrarily shaped metallization patterns. Such a driver is usually composed of arbitrarily shaped metals, arrayed vias, and DMOS transistors. We use FEM to extract the parasitic resistance of the source/drain metals since its strong contribution to Ron. In order to handle the large design case and accelerate the extraction process, a domain decomposition with virtual terminal insertion method is introduced, which succeeds in extraction for a set of industrial test cases including those the FEM without domain decomposition failed in. For a layout in which the DMOS cells are regularly placed, a sub-domain reuse procedure is also proposed, which obtained a dramatic speedup for the extraction. Even without the sub-domain reuse, our method still shows advantage in runtime and memory usage according to the simulation results.