The Al-5Nb-RE-B master alloy with a uniform microstructure has been prepared using melt reaction method. The effect of Al-5Nb-RE-B inoculation on aluminum alloy A356 for their grain refinement has been studied. It was found that the grain size of A356 alloy can be reduced from 800 μm to 200 μm by the addition of 1wt.% of the prepared Al-5Nb-RE-B master alloy. The in-depth analysis of the alloys solidified under wide range of cooling rates indicated that Al-5Nb-RE-B inoculation does not only lead to a much finer microstructural features but also makes the final grain size far less sensitive to the cooling rate employed to solidify the material.