Nanogaps were prepared on the Pd line electrode by focused ion beam, and electron field-emission characteristics of the Pd nanogap emitter subject to hydrogen plasma treatment were studied. The as-prepared nanogap had smooth and uniform gap edges, and thus field-emission characteristics of the nanogap emitter were primarily dependent on the gap separation. After the hydrogen plasma treatment, the field-emission property of the Pd nanogap emitter was significantly enhanced. The improvement in the field-emission property was mainly attributed to formation of a ragged morphology on the nanogap emitter during the hydrogen plasma treatment. The ragged morphology provided more emitting sites with a high field enhancement factor. The Fowler-Nordheim plot was used to elucidate the dependence of field-emission characteristics of the Pd nanogap emitter on the plasma-induced ragged morphology.