High-pressure and high-temperature stability of MgCO3 magnesite was investigated using a lever and spring type of diamond anvil cell up to 55GPa at about 1300°C. No phase transformation was observed in magnesite at the pressures investigated. Magnesite would be a dominant carbon-reservoir in the lower mantle. The volume of magnesite at 55GPa is 77% of that at the atmospheric pressure. The compressibility in the c-axis is 8% larger than that in the a-axis.