An analytic subthreshold potential model for gate underlap cylindrical gate-all-around (GAA) MOSFETs is presented in this work. The fringing field from the gate to underlap regions is derived by using channel length transformation and conformal mapping. The result is then applied to solve the Poisson equation to obtain the subthreshold potential distribution in the channel region of a GAA MOSFET. The model has been verified by extensive three dimensional numerical simulations. (C) 2010 Elsevier Ltd. All rights reserved.