The use of biodiesel in compression ignition (CI) engines has great potential to reduce emissions and the dependence on fossil fuels. However, the complete adoption of biodiesel in CI engines is limited by several drawbacks. A promising solution to these problems is the addition of nanoparticles to biodiesel or diesel-biodiesel blends. Hence, this paper studies the impact of adding aluminum oxide (Al2O3) nanoparticles on the performance and emission characteristics of a diesel engine running on biodiesel and a mixture of diesel and biodiesel. Al2O3 nanoparticles were added at concentrations of 50, 100 and 150 ppm to biodiesel (B100) and a mixture of 50 % biodiesel and 50 % commercial diesel (B50). The results indicated an increase in fuel conversion efficiency and a reduction in specific fuel consumption. The highest fuel conversion efficiency was 26.5 % for B50 with 100 ppm of Al2O3, indicating an improvement of around 28.6 % compared to diesel fuel (B0). In general, CO emissions were reduced with the addition of nanoparticles, with a maximum reduction of 42.8 % for B100 with 50 ppm Al2O3 compared to B0. The highest reduction in NOX emissions was 30.3 % for B50 with 100 ppm Al2O3 compared to pure diesel B0.