The demand of clean energy, like liquefied gas(LNG), increase suddenly because it generates few polluting substances when burned and from the point of view with caloric value it generates ralatively less $CO_2$ gas than the other energy sources. LNG transpotion method of our country is marine transportion by ships because the LNG producing district is far away from Korea. Main engines for most LNG ships are steam turbines, and the efficiency of steam turbine is influenced by the degree of vacuum of main steam condenser. This paper introduce the design method of the vacuum system for high efficiency marine steam turbine. Especially, it is developed the CAD program for the large steam condenser and steam ejector. Also, it is designed the pilot plant including high pressure boiler for the performance test and maked a part of this plant.