By employing Lagrange equation of motion, a dispersion equation that accounts for the growth of axisymmetric, as well as non-axisymmetric waves on an electrically charged liquid jet is derived in this work, with considering the electric field configuration of a needle-plate apparatus for liquid spraying. The available theory is capable of predicting the phenomenon of both of axisymmetric and non-axisymmetric deformations of the liquid jet, which has long been observed in practice. It is demonstrated that there exist certain ranges of such parameters as applied voltage and needle-plate distance where the axisymmetric and non-axisymmetric modes of instability take place. The axisymmetric mode has a dominant effect at low electric field strength. As the electric field strength is increased, the nonaxisymmetric mode is intensified. The present theory indicates that the critical wavelength, and thus, the droplet size are decreased with increasing applied voltage and/or shortening needle-plate distance, which is consistent with the experimental results.