The rotational barriers of BD2 in B2D4 (D=H, F, OH, NH2, and CH3) are examined in terms of conjugation, hyperconjugation, and steric effect at the level of HF/6-31G*. By deleting or keeping the pi orbitals on the boron atoms in the HFSCF calculations, for the first time we are able to analyze the hyperconjugation effect on molecular structures and stabilities quantitatively. In the perpendicular structure of B2H4, hyperconjugation results in the shortening of B-B bond from 1.705 to 1.679 Angstrom and stabilizes the system by 3.1 kcal/mol. For B2D4 systems in which D are pi donors, there is a competition between the steric effect and pi electronic delocalization. The former prefers a perpendicular conformation while the latter prefers a planar one. ForB(2)F(4) and B-2(NH2)(4), these two factors are comparable and therefore, the rotational barrier is close to zero. For B-2(OH)(4), the intramolecular hydrogen bonds stabilize the planar structure significantly, For B-2(CH3)(4), however, steric effect dominates and consequently perpendicular structures are overwhelmingly preferred.