Screw oil pressing is to separate the oil from oil bearing materials by the pressing power which comes from the change of chamber volume, drive from screw worm, resistance of curve teeth at press rings and extrusion friction between oil materials. Under the big pressure created in press chamber, the oil can be discharged from gap of press bars and oil trough of press rings. Either big scale and small scale of screw oil expeller, they have similar theories in screw expeller pressing. Because the screw type seed oil expeller is mainly used by extrusion theory, so it is also called as oil extruder or oil extrusion process.