Borides have been widely used in cast TiAl alloy for grain refinement and a variety of stoichiometry and crystal structure of borides were reported. Here the effects of alloying elements Nb, Ta, and Mn on the structural stability of fine boride precipitates in TiAl alloys have been studied combining transmission electron microscopy (TEM) and first-principles calculations. The results show that most boride particles have the TiB stoichiometry. In the alloy containing Nb and Mn, all the TiB particles have the B27 structure and are highly enriched with Nb but depleted with Mn. In the alloy containing Nb and Ta, however, the intergrowth of Bf and B27 structure has been observed, and the TiB particles are enriched with both Nb and Ta. First-principles calculations reveal different effects of Nb, Ta, and Mn on the structural stability of TiB polymorphs. Nb stabilizes B27 but destabilizes Bf. Ta strongly stabilizes both B27 and Bf structures. Mn strongly destabilizes both B27 and Bf structures. Display omitted •The B27-type TiB in TiAl is enriched with Nb and Ta but depleted with Mn.•Nb stabilizes B27 but destabilizes Bf.•Ta strongly stabilizes both B27 and Bf structures.•Mn strongly destabilizes both B27 and Bf structures.•Atomic structure of the B27/Bf interface has been determined.