Grain refiners are used in the aluminum DC casting process to refine the cast structure. On the other hand, high levels of TiB2 particles and other inclusions from the grain refiner can cause clogging of the melt filter. Improvement of the grain refinement effectiveness is required in order to reduce the addition level of grain refiners. The grain refiner manufacturer AMG developed “TiBAl Advance”, which is a high-performance Al-3Ti-1B (mass%, following is the same) grain refiner. TiBAl Advance has passed a special casting test to provide grain refiners with a high grain refinement efficiency. However, the reason why it shows such high grain refinement effectiveness is yet to be clarified. In this work, the grain refinement effectiveness of TiBAl Advance was compared to that of conventional grain refiners. In addition, the TiB2 agglomerate size distributions were measured in each refiner and applied to the new UACJ model for grain size prediction. TiBAl Advance was demonstrated to have superior grain refinement effectiveness compared with other conventional grain refiners. Furthermore, the predicted grain size from the new UACJ model agreed well with the experimental results. Based on these results, it was estimated that the superior grain refinement effectiveness of TiBAl Advance is due to the smaller size of TiB2 agglomerates, and consequently greater number of heterogeneous nuclei available for any given volume fraction of TiB2.