A new grain refining technique has been firstly put forward for commercial pure aluminum by addition of molten AlTiB master alloys. Comparing with traditional technique, this method has the advantages of shorter contact time and better refining effect, and the difference of refining efficiencies of AlTiB master alloys is greatly reduced. Nucleating sites for a-Al grains have been analyzed after addition of AlTiB master alloy into pure aluminum. When AlTiB master alloy is added in the form of solid, most of TiB2 particles appear in a large TiB2 agglomerate, and the nucleating activation of TiB2 particles is greatly reduced. On the other hand, when AlTiB master alloy is added to pure aluminum in molten and diluted state, the TiB2 particles is separate, and the nucleating activation and nucleating rate of TiB2 particles is greatly increased.