The deformation of the bubbles attached on the heat transfer surface in electric field and its affects on EHD enhancement of boiling heat transfer based on analysis of electric filed distribution investigated theoretically and experimentlly. Bubble deformation depends on electric field ditribution. If electric field intensity of the heat transfer surface is higher than that of the electrode, the bubbles are elongated in the direction of electric field by di-electricphoretic force, otherwise the bubbles are pressed. With the presence of the superheat boundary layer, the bubble elongation effect is decreased and the press effect is increased, but the bubble deformation manners are similar, both of which can enhance boiling heat transfer. However, the mechanism of boilng heat transfer enhancement for elongated bubble is different from that for pressed bubbles.