A unique nitriding technique using the atmospheric-pressure plasma jet is applied to narrow holes to harden the inner wall. The pulsed-arc plasma jet generated with nitrogen-hydrogen gas mixture is sprayed for 2 h onto the tool steel having a through hole of 10 mm in length and 2 and 1 mm in diameter. As a result, the inner wall of 2 mm in diameter is uniformly hardened up to ca. 100 µm in thickness. In the case of inner wall of 1 mm in diameter, however, the thickness of the formed hard layer is decreased with the hole depth. We found that guiding the jet plume into the hole by pumping can improve the uniformity of the layer thickness.