Disclosed is a semiconductor device, and more particularly, a manufacturing method of a high voltage semiconductor device. The method includes: forming a semiconductor substrate having a key area for an alignment key, a low voltage area for a low voltage device, and a high voltage area for a high voltage device; forming an oxide film on the substrate; and forming an insulating film on the oxide film. After removing the insulating film, the method includes forming a plurality of shallow trench isolations (STI's) in the areas of the substrate; forming a nitride layer on the substrate and on STIs; sequentially forming a plurality of wells and drift areas by implanting an impurity ion into the high voltage area; and sequentially forming the plurality of wells and the drift areas by implanting an impurity ion into the low voltage area. A system on chip (SOC) process may thus be simplified.