The surrounding-gate metal oxide semiconductor field effect transistor (MOSFET) provides the best device structure for the control of the short channel effect. In this paper, we present an improved explicit current-voltage (I-V) model for a long-channel undoped surroundinggate MOSFET. The derivation of this new model considers variations in silicon film radius and gate oxide thickness. For important cases of large silicon film radius and small gate oxide thickness, the new model shows a better agreement with a numerical solution than the previous explicit model. The accuracy of the new model is verified by comparison with numerical simulations.