The field enhancement due to the dielectric is theoretically investigated for a simple triple junction of metal-dielectric vacuum. The electric potential near the junction is obtained as a function of geometry, dielectric constant, and position. It is found that the key factor to determine the field enhancement is the ratio of the angles subtended by the dielectric and vacuum portions. This enhancement is attributed to the increase of the surface charge density on the metal around the junction by the polarization charge of dielectric. The image field is also found to decrease or increase according to the geometry of the constituents. The current results give rise to a concept in field emission. (c) 2006 American Vacuum Society.