The lattice dynamics of cubic BaTiO3 and SrTiO3 at different crystal lattice volumes were investigated using first-principles density functionaltheory calculation. The computational results indicate that the Ti–O and A–O(A = Sr or Ba) interactions are responsible for the unstable ferroelectricΓ15 andantiferrodistortive R25 phonons, respectively. With decreasing volume, theΓ15 phononbehaviors of cubic SrTiO3 and BaTiO3 show a similar trend and the Ti–O repulsions are significantly enhanced,leading to the disappearance of ferroelectric instability. However, forBaTiO3 the largeionic radius of Ba with respect to Sr results in a repulsive Ba–O interaction and thereby the disappearanceof the R25 instability, while the long-range characteristic of Sr–O pair are crucial for the antiferrodistortive instabilityof cubic SrTiO3. By analyzing the real-space interatomic force constants, it can be confirmedthat the different antiferrodistortive behaviors of the two compounds aredictated by different short-range repulsions of A–O pairs. Moreover, owingto the smaller ionic radius of Sr relative to Ba, the lattice constant ofSrTiO3 can be significantly reduced, leading to the absence of ferroelectric instability forSrTiO3. The substitution and different crystal volumes make the phase transitions of the twocompounds entirely different.