The lattice distortion under high pressure of an antiferromagnetic material NiO has been investigated within the density-functional formalism with the local-spin-density approximation, using the optimized pseudopotential method and plane-wave basis sets. The result of the calculation shows that the magnetic distortion is enhanced by applying hydrostatic pressure. This distortion becomes extremely large at pressures greater than about 60 GPa. Furthermore, the first-order phase transition from the distorted rocksalt to the cesium-chloride structure is calculated to occur at 318 GPa.