The pressure dependence of the optical phonons in LiNbO3to 21 GPa and in LiTaO3to 10 GPa has been investigated by Raman spectroscopy, using the diamond anvil cell. All the observed modes increase in frequency with pressure, and no mode softening occurs. This is in contrast to the behavior observed in BaTiO3and PbTiO3, which are well‐known displacive‐type ferroelectric. It is suggested that this difference in pressure behavior may stem from a different type of phase transition in LiNbO3and LiTaO3, namely the order‐disorder type.