The outline of Tier 2 and Tier 3 NOx reduction regulations of IMO became clear at the meeting of MEPC57, which was held on this spring. Tier 2 can be achieved using conventional technologies; timing retard, engine tuning, electronically control of fuel injection and water technologies. However, Tier 3 will not allow such technologies, which are not able to achieve the future regulation line. The most closed technology is the after treatment system; SCR (Selective Catalytic NOx Reduction system) . SCR has some problem to install the vessels which are propelled by the 2-stroke cycle low speed diesel engines. The biggest problem is the low temperature of exhaust gas, which is under the 300 °C. In the atmosphere of the exhaust gas with Ammonium under the 300°C, it is generated the acid ammonium sulfate, which coated on the surface of catalyst and decrease the effectiveness of chemical reaction. In conventional trials of SCR on actual vessels, all SCRs were installed before turbochargers in order to avoid the low temperature exhaust gas. However, with SCR installed before turbocharger is not convenient for the partial load operation or coastwise services.