In recent business of new building LNG carriers, demands have frequently been made for a alternative propulsion system to replace the steam turbine propulsion. In the background to this are sharp increases in world-wide LNG demands and the advent of new style of LNG trade, leading to a strong intention to improve transportation costs by introduction of the enlargement of cargo tank capacity and alternative propulsion systems in many new LNG projects. Furthermore, an abrupt increase in number of ships in service has led to a serious shortage of propulsion system operators, who need to be skilled for steam turbine propulsion plant. These occasions in recent years stimulated the development of new techniques represented by the boil-off-gas (BOG) re-liquefaction plant and the duel fuel gas engine has been put into practical use in quick succession. Developments of alternative propulsion system are now being promoted mainly focused on diesel engine application, while Mitsubishi Heavy Industries, Ltd. (MHI) is going ahead with the development of various types of propulsion systems. This paper introduces the HYBRID LNG system originated and developed by MHI, making mention also of the latest trends in substitute propulsion systems.