The aim of this experimental work was to explore the effects of SCR System on NOx reduction in heavy duty diesel engine fuelled with diesel and alcohol blends. The experimental tests were performed in a 6-cylinder, turbocharged heavy duty diesel engine at full load. In the experimental tests diesel, ethanol, methanol and butanol were used as fuel. The alcohol fuel blends were prepared by mixing low sulphur diesel at volumetric rates of between 5 to 15%. The test results showed that SCR system reduce the NOx emissions 42.6% for diesel fuel. The maximum NOx reduction (43.43%) was achieved with 15% methanol–85% diesel fuel (D85M15) blend.