In this work we propose an analytical model for the threshold voltage (V-T) of III-V cylindrical nanowires, that takes into consideration the two dimensional quantum confinement of the carriers, the Fermi-Dirac statistics, the wave-function penetration into the gate insulator and the non-parabolicity of the conduction band structure. A simple expression for V-T is obtained assuming some suitable approximations. The model results are compared to those of a 2D self consistent Schrodinger-Poisson solver, demonstrating a good fit for different III-V materials, insulator thicknesses and nanowire sizes with diameter down to 5 nm. The V-T dependence on the confinement effective mass is discussed. The different contributions to V-T are analyzed showing significant variations among different III-V materials. (C) 2013 Elsevier Ltd. All rights reserved.