The high-pressure behavior of ZrO2 is studied using an ab initio constant-pressure technique up to 140 GPa. Two high-pressure phases of ZrO2 are predicted through constant-pressure simulations. ZrO2 undergoes a first-order phase transformation from the baddeleyite structure to an orthorhombic structure with space group Pbcm at 35-40 GPa. The coordination number of Zr atoms unexpectedly changes from sevenfold to sixfold owing to this phase transformation. Further increase in pressure leads to another first-order phase transformation from the Pbcm structure to a tetragonal one with the space group P4/nmm at 70-80 GPa. In this structure, Zr atoms are ninefold coordinated. These phase transformations should occur around 11 and 38 GPa from enthalpy calculations, respectively.