Under cooling rates of 2 ℃/s and 10 ℃/s, the influences of B content on the microstructure of near eutectic Al-13.0wt%Si alloy have been investigated. Results showed that the addition of boron resulted in refinement of eutectic grains, and to some extent, had an inhibiting effect on precipitation of the primary phases, and the refining and inhibiting effects are much more obvious at higher cooling rate. When B was not added, higher cooling rate promoted the a-Al dendrites formation. At lower cooling rate, the addition of B did not cause the so called “columnar to equiaxed transition (CET)” , however, at higher cooling rate, this transition was obvious. After the addition of B, the nucleation temperature TN ascended and nucleation mode changed from nucleation mode of from wall towards centre (without B addition) to a nucleation mode that the eutectic nucleated evenly throughout whole sample (with B added). It can be concluded that the addition of B offers a large amount of nuclei for eutectic solidification, as a result, the eutectic grains was refined. Higher cooling rate will lead to more nuclei, so the effects on the refinement of eutectic grains and on suppression of primary phases are increased. Results showed that the addition of boron resulted in refinement of eutectic grains, and to some extent, had an inhibiting effect on precipitation of the primary phases, and the refining and inhibiting effects are much more obvious at higher cooling rate. When B was not added, higher cooling rate promoted the a-Al dendrites formation. At lower cooling rate, the addition of B did not cause the so called “columnar to equiaxed transition (CET) ”, however, at higher cooling rate, this transition was obvious. After the addition of B, the nucleation temperature TN ascended and nucleation mode changed from nucleation mode of from wall towards center (without B addition) to a nucleation mode that the eutectic nucleated evenly throughout whole sample (with B added). It can be concluded that the addition of B offers a large amount of nuclei for eutectic solidification, as a result, the eutectic grains was refined. higher cooling rate will lead to more nuclei, so the effects on the refinement of eutectic grains and on suppression of primary phases are increased.