An energy saving is the most important subject in planning propulsion plants of merchant ships. In the future, however, measures for the global environment must be added to the energy saving. That is, carbon dioxide (CO₂ gas of greenhouse effect) and nitrogen oxides and sulphur oxides (NOx • SOx, gases of air pollution and asid rain) must be reduced to maintain the global environment. In this paper, influences of additional costs for reduction of these gases are investigated in LNG carriers, in which various propulsion plants such as steam turbines, diesel engines are studied in parallel today. Three major plants are investigated; steam turbine (S/T), dual fuel diesel (DFD) and oil burning diesel with reliquefaction plant (DRL). Firstly generation of CO₂, NOx and SOx gases during operation is calculated for each plant and fuel (heavy oil and boil-off gas). Secondly costs for reduction of these gases are investigated, that is, carbon tax is imposed for CO₂ generation, NOx reduction cost is estimated for diesel, and low-sulphur heavy oil is used in coastal voyages. Calculation results of transportation cost (T/C) with these additional costs indicate that T/C will increase by 3-4% in each plant and T/C of S/T is minimum. That means that S/T will be optimum even if costs for the global environment is added in the future. In tankers, bulkers, etc. whose production prices are much lower than those of LNG carriers, T/C will increase by 6-8%, 10% or even more. Carbon tax is estimated to be very high while costs for NOx and SOx reduction are not so high. In this connection, energy saving and development of new fuels and new power plants must be accelerated. The CO₂ problem is much bigger in land plants. Therefore, technology on CO₂ fixation in marine farm or meadow, CO₂ storage in deep sea, etc. must be developed by marine engineer.