The behavior of electrolytic gas bubbles and their effect on the cell voltage in water electrolysis were studied theoretically and experimentally. A fundamental force analysis was employed to predict the critical diameter for the departure of the electrolytic gas bubbles. Good agreement between the predictions and observations was obtained. It was found that increasing the electrode potential strengthened the force due to the interfacial tension and increased the critical diameter, while increasing the electrolyte concentration led to a reduction. This was explained by the changes in both the contact angle and surface tension. Many more fine gas bubbles were observed at high current density, which was explained by that the enhanced natural convection forced bubbles to depart prematurely. The cell voltage was only slightly reduced by the electrolyte circulation, which reduced the critical diameter for bubble departure. This confirmed that the layer of fine bubbles represented a significant energy barrier, that is, the additional resistance due to the bubble curtain formed on the electrodes, for alkaline water electrolysis.