<p>Gas turbine combined systems using natural gas as fuel can be used as ship propulsion systems, given their low NOx and zero SOx emission characteristics. In this paper, the authors examined their adaptability to ships by comparing them with diesel engines that meet both NOx and SOx emission requirements, from the viewpoints of their sizes and initial costs. Up until now, gas turbine combined systems have rarely been used for ships, and NOx and SOx reduction systems for diesel engines have not been widely utilized. For these reasons, every available information about these systems has been used to examine the case. The results found that gas turbine combined systems using natural gas as fuel required much less space than the diesel engines and that the initial cost of the gas turbine combined systems per unit power was somewhere between that of a diesel engine with NOx and SOx reduction systems and that of a diesel engine with a NOx reduction system using low sulfur fuel.</p>