Effects of P modification on primary Si in hypereutectic Al-Si die casting alloy with different cooling rates were investigated. It is revealed that the tendency of size decrease of primary Si in hypereutectic Al-Si alloy after and before P modification is very similar with increasing in cooling rate

where both have a sharp transition zone. P modification can greatly promote the refinement of the primary Si in the alloy with different cooling rates

because there exists a region between two change curves of primary Si size after and before P modification. Under the same cooling rate conditions

the refinement effects can be improved by 42.7% after P modification

and P still as good modification effects on primary Si with high cooling rate （ casting thickness of 1mm）. The size of primary Si can be significantly refined with P modification under rapid solidification conditions. Al-P master alloy is an excellent modifier for hypereutectic Al-Si die casting alloy.