Hugoniot compression curve of w¿stite (Fe1--xO) was studied up to 120 GPa using a double-stage light-gas gun. Pore-free polycrystalline w¿tite was grown by the floating zone method and platelet specimens, typically 10¿14¿2 mm3, were prepared. The inclined mirror method was adopted to record the shock compression state and both the impedance matching solution and the free surface approximation were used to deduce the Hugoniot compression curve. A discontinuous increase in density was observed at around 70 GPa. This result is consistent with the previous shock compression experiment using hotpressed specimen. The nature of this discontinuity is discussed. ¿ American Geophysical Union 1988