The structure and equation of state for CaIrO3-type MgSiO3 were determined using high-pressure powder X-ray diffraction from 116 to 144 GPa. The CaIrO3-type phase remained stable over this entire pressure range. At each pressure increment, the sample was heated with a laser to relax the deviatoric stress in the sample. Pressure-volume data could be fitted to the Birch-Murnaghan equation of state with K0 = 237(1) GPa using Anderson’s gold pressure standard, when K0’ and V0 were set to 4 and 162.86 Å3, respectively. This indicates that the bulk modulus of MgSiO3 decreases when the phase transition occurs, because the bulk modulus of perovskite-type MgSiO3 is 250–260 GPa. The b axis of the unit-cell parameter was more compressive than the a and c axes. The unit-cell volumes at high pressures, observed in high-pressure experiments, were slightly smaller than those predicted by theoretical studies.

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