The refining effect of Al5Ti1B0.5RE master alloy on A356 Al alloy was studied, and microstructures of A356 Al alloy and the master alloy were analysed by OM、SEM、EDAX etc. The results showed that the master alloy had not only obvious effect of grain refinement, also a significant role in refining dendrite grain of A356 Al alloy. RE elements played an important role in the master alloy, that is, RE could easily form （AlTiRE） phase in the master alloy, but （AlTiRE） phase was unstable in Al melt and could dissolve quickly by releasing RE elements, which deceased the surface energy of Al melt and the tendency of TiB2 particles getting together in Al melt, thus increasing. the number of nuclear particles. Moreover, RE elements could easily get adsorption together between the grain boundary and the phase boundary of Al alloy, thus filling the defects of grain surface, and hindering TiAl3 growing and the formation of needle or strip TiAl3 phase, which increased the nucleation rate on refinement nuclei. Furthermore, RE could effectively guarantee metallurgical quality.of the grain refiner and Al melt, thus also enhancing the effect of the grain refiner.