Air pollution from ships is increasing due to higher global demand of goods, and with them, global emission standards as well. These standards are prescribed by MARPOL Annex VI and the introduction of emission control areas (ECA). Ships built before 1990. do not have to comply with these rules, but ships built after the early 2000. must follow “NOx Emission Tier I”, which allows 17g/kWh NOx (for the slow speed diesel engines with <130 rpm), and selective catalytic reactors (SCR) are not necessary. Tier III NOx Emission standard was enforced in 2016 and it requires an 80% reduction in NOx emission compared to the Tier I, specifically 3.4 g/kWh, which can not be achieved without SCR or without using some other types of fuels. The focus of this paper is to do analysis how the ammonia dosage to SCR affects to the NOx formation in exhaust gasses on various engine speed (loads), as well as how it affects to some important engine parameters and specific fuel oil consumption (SFOC). The analysis was performed on Kongsberg engine room simulator (ERS), MC 90-V Model, as well as on MATLAB by using mathematical models. All external influences are excluded from the analysis like wind or rough sea. All data on different scenarios has been recorded after prolonged period to avoid inconsistencies. The results are shown graphically, and conclusions are presented.