The pressure effects on the oxidation of Zircaloy-4 (Zry-4) cladding in high temperature steam have been analyzed. A double layer autoclave was made for the high pressure/high temperature oxidation tests. The experimental test temperature range was 700-900 o C, and pressures were 0.1-15 MPa. Steam partial pressure was found to be a more important factor than total pressure. Steam pressure enhances the oxidation rate of Zry-4 exponentially and depends on the temperature, the maximum being between 750-800 o C. A preexisting oxide layer decreases the enhancement by about 40-60%. The acceleration of the oxidation rate caused by high pressure steam seems to originate from the formation of cracks due to an abrupt transformation of the tetragonal phase in the oxide, where the instability of the tetragonal phase comes from the reduction of surface energy due to steam.