In this study, the effect of biodiesel premixing on emission, combustion characteristics and engine performance was numerically investigated via AVL Fire. In order to confirm the validity of the study, combustion inside the chamber was simulated in a condition in which the fuel was pure diesel, and the results were compared with results presented in other work. Engine performance parameters and emissions were in a very good agreement with those reported in other research works. After the validity of the engine achieved, in the first phase of the study, pure diesel fuel and the mixture of diesel-biodiesel blends were injected to the chamber without premixing. In the next phase, pure biodiesel and biodiesel blends were premixed within the chamber and the effect of premixing on the interesting parameters was investigated. Results showed that premixing biodiesel leads to a reduction in the soot emission and increment in the NOx emission. In addition to this, using pure diesel as the main injected fuel and pure biodiesel as the premixed fuel decreased soot emission considerably. Besides, unlike biodiesel blends, it did not increase NOx emission considerably. (C) 2019 Elsevier Ltd. All rights reserved.