A complete carrier-based non-charge-sheet analytic theory for the nano-scale undoped surrounding-gate MOSFETs is presented in this paper based on the basic device physics. The formulation is based on the Poisson?s equation to solve directly for the mobile carrier-the electron concentration. Therefore, the distribution of the potential, the field and the charge density in the channel away from the surface is also expressed in terms of the carrier concentration, giving a carrier-based non-chargesheet model for nano-scale undoped surrounding-gate MOSFETs including the short-channel effects. The formulated theory has an analytic form that does not need to solve the transcendent equation as in the conventional surface potential model or classical Pao-Sah formulation. As a result, the theory can analytically predict the analytical IV and CV characteristics of the undoped surrounding-gate MOSFETs. The validity of the theory results has also been demonstrated by extensive comparison with 3-D numerical simulation.