In a general plane-parallel electrode system the edge of the electrode will undermine the uniformity of the dielectric barrier discharge （DBD） because of the influence of the distorted electrical field. In this paper, the influence of the non-uniform electrical field on the edge effect of DBDs in a short-gap is investigated. We present some o.f the experimental results of DBDs produced by three kinds of convex-spherical electrodes. The results show that there is a dark area （the homogeneous discharge） in the central region of the electrode and a bright halo （the filamentary discharge） in the outer peripheral region, and the radius of the dark region is determined by the electrode geometry. The calculated results of the transverse （radial） field component distribution on the surface of the electrodes show that the edge effect does not come from the electrode edge, but the transverse field. The discharge has enough space to be fully developed and then format the filamentary discharge in the outer peripheral region because the streamer of the filamentary discharge is driven to move along the direction of the longer path by the transverse field. Thus, the homogeneous discharge （the Townsend DBD or a glow DBD） could not be produced in this area.
