One of the most important exhaust emissions in diesel engines is nitrogen ox-ides (NOX). The most effective method in reducing NOX emissions is Selective Catalytic Reduction (SCR) systems. Although it has been used in light and heavy-duty vehicles in Turkey for about 15 years, it has become used in all die-sel motor vehicles, including cars as of 2020 due to the changing emission regu-lations. In the SCR system, nitrogen oxide emissions are reduced to nitrogen and water by injection of the diesel exhaust fluid (DEF) to the catalytic reactor. In this study, the SCR system was installed in a single-cylinder diesel engine and the DEF injector characteristic was extracted and NOx conversion efficiency was examined in various injection quantities. The aim of this study is to provide the best NOx conversion by finding the optimum injection amount and to reduce the accumulation of urea in the system by obtaining minimum DEF consumption. In experimental studies, NOx conversion was examined according to the DEF con-sumed in various injection amounts of 40, 80, 160, 240, 320, 400 and 560 mg/s. In order to determine the minimum DEF and maximum NOx percentage reduc-tion, the reduction % in NOx emission was proportioned by the DEF injector mass flow rate (mg/s). The highest result was obtained with a value of 0.38 with 40 mg/s injection.