By means of hybrid density functional theory,we interpret the stability mechanism of the tetragonal CuO phase,which was synthesized using the pulsed laser deposition.The orbital ordering resulted from the crystal field splitting is found to be favorable for the d9 electronic configuration of the Cu2+ ion,yielding two possible metastable tetragonal phases (c/a ＜ 1 and c/a ＞ 1) of CuO.A detailed comparison is also performed with the ideal rock-salt compounds CoO and NiO.