The effects of boron on the grain refinement of Al alloys are investigated. Small addition of boron has no effects on the grain refinement of pure aluminum and Si free Al alloys. These alloys are, however, well grain-refined with low Si content of 0.1 similar to 0.5mass%. The binary hypoeutectic Al-Si alloys are also refined by boron addition independently on the Si concentration. The grain refining effects are kept after long holding times, high holding temperature on the melt conditions and remelting. The boride particles in the Al-B master alloy do not act as effective nuclei of alpha -Al and it is doubtful whether the heterogeneous nucleation of alpha-Al occurs directly on AlB2. The grain refinement mechanism should be focused on the reactions of Al-Si-B ternary system.