Based on the equilibrium structure obtained, the ground states of ZnO molecule under external electric fields ranging from -0.05 to 0.05 a.u. were optimized using the density functional theory B3P86 at 6-311++g(d,p) level. Effects of electric fields on the bond length, total energy, charge distribution, energy levels, HOMO-LUMO gap and the infrared spectrum of the ground states of ZnO molecule have been investigated systematically. The results show that the molecular geometry and electronic properties were dependent on the magnitude and direction of the external electric field considerebly. With the increase of electric field along the molecular axis O-Zn, the equilibrium bond length first decreased and then increased, while the total energy, the harmonic frequency and infrared spectrum first increased and then decreased. But the HOMO, LUMO energy levels and the energy gap decreased monotonically, indicateing that the molecule could be excited easily by a specific electric field. We think that the present results are useful for better understanding the physical mechanism underlying the electroluminescence properties of ZnO molecule.