The crystal structure of the CaO compound is studied up to 300 GPa under high hydrostatic pressure using the density functional theory (DFT) with the generalized gradient approximation (GGA). Pressure-volume relationships, structural transitions and electronic properties in CaO compound are investigated using Siesta method. CaO crystallizes in the NaCl-type (B1) structure (space group: Fm (3) over barm) in ambient conditions. CaO transforms from this structure to CsCl-type (B2) structure (space group: Pm (3) over barm) at high pressure. This transformation is based on a intermediate state with space group R (3) over barm. Moreover, the electronic band structures of the B1 and B2 structures of CaO have been calculated. According to this calculation, obtained band gap values are in good agreement with the values reported in the literature.