The present invention discloses a double diffused metal oxide semiconductor (DMOS) device and a manufacturing method thereof. The DMOS device is formed in a first conductive type substrate, and includes a second conductive type high voltage well, a field oxide region, a gate, a second conductive type source, a second conductive type drain, a first conductive type body region, and a first conductive type deep well. The deep well is formed beneath and adjacent to the high voltage well in a vertical direction. The deep well and the high voltage well are defined by a same lithography process step.