The research for renewable and less polluting fuels has focused on biodiesel. This fuel can derive from vegetable, animal or waste oils, and despite its potential to decrease atmospheric pollutants and greenhouse gases, its influence on NOx emissions is still uncertain. It is believed that biodiesel emissions, especially NOx vary depending on the feedstock, blend percentage and transesterification route. A better understanding of these factors can help choosing the best blend. In this context, this article aims at evaluating how the variation of these factors affects NOx emissions. Tests are carried out in a stationary internal combustion engine with 20% and 50% blends of methyl and ethyl esters made from soybean oil, palm oil and waste frying oil (collected in the University Campus). The analysis of the results with Tukey's test compare their means and lead to the conclusion that, when considering the route, ethyl blends have lower NOx emissions, and palm ethyl ester blends had the best results. Also when all factors are taken into account, B20 from soybean methyl ester has the lowest emissions of NOx. We recommend that future studies test the effect of antioxidants in NOx emissions, as well as test higher blend ratios. (C) 2016 Elsevier Ltd. All rights reserved.