Compression mechanisms of A2+B4+O3 ilmenites with compositions MgSiO3 (stable at high pressures), MgGeO3 (stable at moderate pressures), and MgTiO3 (stable at ambient pressure) were investigated at high pressure by single-crystal structure analysis, using both synchrotron radiation and an MoKα rotating-anode X-ray generator. The distortions of AO6 (A:Mg) and BO6 (B: Si,Ge,Ti) octahedra under pressure were parameterized by bond length, shared-face area, site-volume ratio, and A2+-B4+ interatomic distance across the shared edges and shared face. The AO6 octahedral volume is much more compressive than the BO6 octahedral volume. Of the three samples, both the AO6 and BO6 octahedra are most rigid in MgSiO3. The A2+-B4+ interatomic distance becomes more shortened with increasing pressure than do the A2+-A2+ and B4+-B4+ distances. The compression of Mg-Si is more remarkable than that of Mg-Ge and Mg-Ti. The A-B interatomic distance along c is more compressed with increasing pressure than A-A and B-B along a. The short A-B distance across the shared face becomes more shortened than the A-A and B-B distances across the shared edge. The cation position moves in the direction of c with pressure and tends to approach the center of the AO6 and BO3 octahedra with increasing pressure. The regularity of the octahedra is enhanced at higher pressure. Both quadratic elongation and bond angle variance verify the reduction of the deformation of AO6 and BO6 octahedra with pressure.