To obtain better microstructures and improve refining performance, a new aluminum grain refiner Al-Ti-B-RE master alloy was prepared by a mix molten-pure titanium method. The results show that there are small blocks of Al3Ti and Ti2A120RE phases in the Al-Ti-B-RE master alloys prepared at optimal temperatures while there are large flakes at high temperature. The morphology, distribution and quantity of second phases of Al-Ti-B-RE master alloys and refining effects and nucleating potentials were studied by XRD, SEM and OM. It is indicated that the morphology, distribution and quantity of second phase particles of Al-Ti-B-RE master alloys have an important influence on the refining effects. A better refining effect could be obtained with the smaller disregistry between nucleation and the substrate in favor of promoting nucleation