In order to meet the IMO NOx TierIII regulations which took effect in 2016, NOx emissions from ships operating in emission control areas (ECA) need to decrease by 80% from the IMO NOx TierⅠlevel. EGR and SCR technologies are believed to be highly effective in satisfying the TierIII requirements. However, according to data obtained with the use of MAN Diesel & Turbo’s Computerized Engine Application System (CEAS), SCR techniques are more fuel efficient than EGR technologies. Especially, the marine SCR system developed by Hitachi Zosen Corporation offers strong fuel economy in that the system can raise the SCR operating temperature above the level required for denitration reaction with exhaust gas energy only. This paper outlines SCR systems and explains the influence of SCR operations on engine performance and fuel oil consumption in a large two-stroke engine 6S46MC-C7 fitted with our High Pressure SCR (HP-SCR).