Global warming is one of serious global problems. According to the presumption of IMO, the amount of the greenhouse gas emissions from international maritime transportation represent 850 million tons in 2007. Furthermore, carbon dioxide emissions from international maritime transportation represent 3% of the world's total carbon-dioxide emissions. Therefore, it is necessary to reduce of carbon dioxide emissions from maritime transportation. In this study, under the background of China's iron ore imports increased rapidly, the reduction effect of carbon-dioxide emissions by enlargement of a vessel is examined. In addition, Yangtze Estuary Deepwater channel in China is shallow and large vessels cannot pass which are fully loaded. Therefore, inquire into the reduction effect of carbon dioxide by enlargement of a ship, taking change of the draft by the loading state of a vessel into consideration. From restrictions of depth of water, enlargement of ship's size reduces a loading ratio, and it was understood that a medium-sized ship is desirable over coastal service transportation. In addition, as a result of verifying about the practical use effect of the tide in coastal service transportation, the loading ratio could be increased ten percent or more, and it turned out that it is effective in reduction of carbon dioxide emissions.
