Impact of a single trapped charge in an oxide layer on the carrier transport is numerically investigated for gate-allaround nanowire (GAA NW) channels, using the ensemble Monte Carlo / molecular dynamics simulation. The relative amplitude of current reduction caused by a single trapped charge increases in smaller NWs. This is due to the closer mean distance of carriers and the trapped charge in a downsized device, resulting increased impact of Coulomb scattering from the trapped charge to carriers. Under the same trap density, however, the smaller NWs show a decreasing tendency of the relative amplitude of current reduction if the current per NW is the same. Even in the practical operation conditions in which the current density per unit width is kept constant for each channel diameter, the relative amplitude of current reduction does not depend on the NW diameter. This indicates that the impact of the trapped charge on the relative amplitude of current reduction is determined by the trap density.