We demonstrate dopant profiling in ultrathin channels (UTCs) (T(c) = 18-58 nm) of vertical-type double-gate metal-oxide-semiconductor field-effect-transistors (DG MOSFET) by scanning nonlinear dielectric microscopy (SNDM). The vertical UTCs were fabricated by orientation-dependent-wet etching. Using ion implantation technology and subsequent furnace annealing, n(+)-p junctions, which correspond to the source/drain of the vertical-type DG MOSFET, were formed in the upper part of the UTC. To improve the accuracy of the vertical dopant profile in the UTC, the cross-section of the UTC was magnified by beveling with a small angle by chemical mechanical polishing. Using such a beveled sample, the dopant depth profile in the vertical UTC has been measured by SNDM with nanometer-scale resolution. On the basis of the measurements of the dopant profile, an effective channel length for the vertical DG MOSFET has also been estimated quantitatively.