An experiment was carried out to confirm the an effect of electric field on the onset of instability of a liquid surface. A static electric field was applied between the liquid surface and a horizontal planar electrode placed over the surface. The applied electric voltage was raised gradually until the liquid surface became unstable. The critical voltages were measured for different distances between the electrode and the liquid surface and for different liquids. Instability criteria were derived theoretically using a modified Rayleigh-Taylor instability equation and it was found that the theoretical results agreed very well with the experimental data. Also, consideration was given to clarify the mechanism of the EHD enhancement of the evaporation of a drop which was reported previously.