The exhaust gases from marine Diesel engines contain harmful substances such as NOx, SOx and particulate matter. These harmful substances cause adverse public health, acid rain and photochemical smog. Taking into account the aforementioned background, strengthening of regulation on the limit of NOx emission have been being discussed at the International Maritime Organization (IMO) and the standard set forth in regulation 13.5.1.1 of MARPOL Annex VI which requires 80 % NOx reduction (Tier III) is applied to the operation of a marine Diesel engine installed on a ship constructed on or after 1 January 2016. To meet Tier III, the Selective Catalytic Reduction (SCR) system is essential in addition to primary NOx reduction measures meeting Tier II. We have conducted research and development on urea based SCR systems for 4-stroke marine Diesel engines to establish practicable NOx reduction technologies. This paper reports the result of catalyst block deterioration tests, numerical analyses on reducing agent spray and on the catalyst deterioration, and the onboard test for the SCR system.