© 2017, Japan Society of High Pressure Science and Technology. All rights reserved. The Hugoniot of magnesium oxide (MgO) has been investigated experimentally and computationally at the pressure range between 200 GPa and 1 TPa, where B1 and B2 structures are theoretically stable and liquid exists. The shock velocity is found to be proportional with the particle velocity up to the shock velocity of 15 km s -1 , suggesting MgO is in the B1 structure up to a shock pressure of ~350 GPa. Moreover, the Hugoniot data combined with ab initio calculations show two crossovers between the Hugoniot and an extrapolation by the linear relation around 350 GPa (and 8000 K) and 650 GPa (and 14000 K), respectively. The regions separated by the two crossovers are interpreted to be the fields of B1-B2, and B2-liquid transitions, respectively. They are consistent with the results by the ab initio calculations.