Hemispherical bubbles composed of smectic liquid crystals show an unusual deformation under DC electric field. When the lower voltage than a threshold is applied to the bubble on a substrate, it is elongated in the direction of the electric field and forms a semi-elliptical shape in the equilibrium. With the higher voltage than the threshold, the bubble becomes unstable and sometimes exhibits a periodic oscillation with repeating the elongation and shrinkage. Based on a simple equation of force balance, we analyzed both the static and dynamic deformations under DC field and successfully reproduced the observed behaviour. By the detailed analysis, we also determined the surface tension and the electric conductivity of the smectic bubbles.