In tankers and bulkers, energy savings made remarkable progress after the oil crisis by means of size enlargement, speed reduction, propeller rpm reduction or switching the main propulsion plant from steam turbine to diesel engine. However, in LNG carriers, which are high-technology and high-price ships, there are many commercial requirements, and size enlargement, speed reduction or propeller rpm reduction, therefore, are limited. Switching to a diesel engine is feasible, but this is more difficult in LNGCs because boil-off gas is generated during voyages and must be properly treated. Also the boil-off gas price is very uncertain. This paper studies diesel LNGCs, analyses uncertainties, investigates the real condition of LNGC operation, and makes clear an energy saving of LNGCs. The conclusion states that turbine driven LNGCs with lower boil-off rate and forced vaporization system are best in terms of economics and operational flexibility. This design was actualized in the Northwest Shelf and Badak III LNGCs and proved to be best as predicted. All LNGCs ordered thereafter were designed in identical concept to the above, which means the world standard of LNGCs has been renovated.