The effect of urea injection on NOx emissions in a six-cylinder, four-stroke internal combustion engine fueled with B80 biodiesel has been experimentally investigated. The results reveal that urea injection leads to a reduction of NOx emissions of B80 biodiesel fuel. Moreover, the influence of injection parameters on NOx reductions has been studied. The findings show that increasing the injection temperature results in lower NOx emissions. Also urea mass flow rate increment leads to more NOx reduction. The same result has been obtained by an increase in spray angle. In addition, increasing the number of nozzles indicates more depletion in NOx emissions while a "three-nozzles spray system" has the most considerable effect on NOx reductions. (C) 2015 Energy Institute. Published by Elsevier Ltd. All rights reserved.