By vacuum arc melting, four kinds of rare earth (RE) elements, Ce, La, Nd and Gd, were added in the range from 0.05% to 0.5% (mole fraction) to binary Ti-44% Al alloy to refine its grains of structures for high ductility. Effects of RE elements on structures and grain sizes of the alloy were investigated and compared. It was observed that Ce had the strongest effect on promoting growth of columnar colonies and Gd had the weakest one, while the other RE elements obviously promoted this growth in as-cast structures of the ingots. It was seen that the four RE elements initially reacted with oxygen to form oxides in the melt, the RE aluminides occur at boundaries during heat treatment, and sometimes the aluminide co-existed with oxide at boundaries or in grains. Results of measurement of grain sizes showed that the grains in the ingots heat-treated were effectively refined by properly adding the four RE elements, and the average grain size in the ingot with 0.15% Gd is finer and more uniform than that with Ce, La and Nd. The results of comparison verified that the sequence of effects of the four RE elements is Ce, La, Nd and Gd from strong to weak, which means that the alloy is sensitive to a small amount of Ce addition and is comparatively dull to a larger amount of Gd.