We have investigated the electric field effect on horseshoe-shape carbon nanotubes (CNTs) resulting from hydrogen adsorption on the single-wall armchair (n,n)CNTs with 6 <= n <= 16 by using the density functional theory calculations. The horseshoe-shape CNT is completely unzipped into a graphene nanoribbon upon applying a critical electric field, which decreases with increasing CNT diameter, thus enabling one to select a nanoribbon width. A simple model based on the tensile force exerted on the tube walls by the applied electric field was introduced to understand the CNT-diameter dependence of the critical field. (C) 2013 Elsevier B. V. All rights reserved.