The optimum configuration is found of a transmutation reactor based on a tokamak neutron source for an aspect ratio in the range of 1.5-4.0 and fusion power in the range of 100-500 MW, by means of coupled analysis of the tokamak systems and the neutron transport. The dependence of the transmutation characteristics upon the aspect ratio is investigated for the case in which the transmutation reactor produces constant power. For a low-aspect-ratio tokamak, the power density requirement was satisfied with less fusion power, and less radial thickness of the breeding blanket was required to meet a tritium breeding requirement, relative to a high-aspect-ratio tokamak. It was shown that by adjusting the portion of minor actinides contained in the transuranic actinides, the nuclear waste transmutation could be optimized with low fusion power while producing constant power. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.