NOx emission is one of the most important pollutants and held responsible for damages on human health and environment worldwide. Undoubtedly, diesel engines, which are widely used in many fields, are the most important sources of NOx emissions. Currently, SCR technology is the most effective method used to eliminate NOx emissions from diesel engines. Thanks to SCR technology, NOx emissions can be eliminated at very high rates by using a reductant and a catalyst. The NOx conversion efficiency of SCR system can be significantly improved using a DOC before the SCR system. This study focused on the effect of biodiesel, which is the primary fuel that can be an alternative to diesel fuel, on DOC and SCR efficiency. In the study, besides the traditional SCR catalyst (V2O5-WO3/TiO2), Ag/Al2O3 catalyst was also tested. The effect of using biodiesel and Ag/Al2O3 catalyst on NOx conversion performance especially at lean conditions is investigated in detail. In addition, the conversion performance of the DOC used in the tests was also examined in this experimental study. The results showed that biodiesel promoted the activity of DOC. The use of biodiesel increased the DOC conversion up to 14.66% compared to diesel while caused no significant change in NOx conversion of SCR catalyst. Maximum conversion rates of Va/SCR and Ag/SCR catalysts were obtained as 70.25% and 70.48% respectively with diesel.