By using different types of processing units such as Crude Distillation Unit(CDU),Vacuum Distillation Unit(VDU),DieselHydrotreating Unit(DHU),we are refining the crude oil.Petroleum refining is the recovery generation of usable fractionsand products from crude oil,either by distillation or by chemical reaction of the crude oil constituents under theeffects of heat and pressure. Petroleum refining also involves treating the raw products by a variety of finishing processes to yield marketable products. A refinery is an integrated group of manufacturing plants that vary in number according to the variety of products produced; it must be flexible and able to change operations as needed. Refinery processes must be selected to produce products according to demand. This article deals with the processes used to refine petroleum.