A new threshold voltage formula is derived for a U-shaped gate MOSFET where the gate oxide becomes thicker at both channel ends. It is found that when charge-sharing effect due to source/drain-junctions is not accounted for, threshold voltage increases with decreasing channel length, exhibiting an inverse short-channel effect (ISCE). When charge-sharing effect is taken into account, the ISCE reduces remarkably, resulting in a flat threshold voltage characteristic over a wide range of channel lengths. Computer simulations using a two-dimensional device simulator show a good agreement with these analytical findings.