The mechanical properties of centrifuged Al-Si-B composites containing aluminum diboride and aluminum dodecaboride particles were studied using optical and scanning electron microscopy (SEM), hardness tests, and nanoindentation. SEM was used to further analyze the Rockwell indentations, while nanoindentation experiments were completed on the matrix and the reinforcing particles. The outcomes of this study were compared with the results obtained in superfi cial Rockwell hardness and Vickers microhardness tests to determine which of the boride reinforcements was more benefi cial for the mechanical properties of the Al-Si matrix composite. Aluminum diboride was found to improve bulk mechanical properties more than did aluminum dodecaboride particles. On the other hand, aluminum dodecaboride showed to be a better reinforcement at the micron and submicron scales, as revealed by nanoindentation experiments. A discussion of these fi ndings helped understand the underlying mechanisms behind this apparent discrepancy.