In this work, the crystal structure of the ZnO was studied under high hydrostatic pressure using ab initio calculations. Pressure-volume relationships and structural transitions in ZnO were investigated using Siesta method. A first-order transition from the hexagonal wurtzite (B4) structure with space group P63mc to the cubic NaCl (B1) structure with space group Fm3m was successfully observed. A transition was also observed from Fm3m to another cubic CsCl (B2) structure with space group Pm 3m for ZnO. These phase transitions which occur around 9 and 119.5GPa were also analyzed from the total energy and enthalpy calculations. In addition, electronic, elastic and vibrational properties of ZnO were analyzed based on the high pressure.