To achieve uniform curing of resin, internal heating with the addition of carbon nanotubes (CNTs) has attracted considerable attention. Numerical simulations of the residual stress in CNT-filled resins cured by an electric field were carried out in the present study, taking into account the CNT dispersion within the resins. The simulations were based on an unsteady-heat-transfer equation and the cure reaction; the residual stress due to the thermal expansion and cure shrinkage was calculated using a finite element method. In addition, microscope images of actual CNT-filled resins were used for modeling the inhomogeneous electrical conductivity due to CNT aggregates. The simulation results show that, compared to external heating, Joule heating, or resistive heating, in which a conductive material itself generates heat from the passage of an electric current, enables more uniform curing and generally suppresses the residual stress. However, high local residual stress was observed around the high-electrical conductivity region in the model with inhomogeneous electrical conductivity. The present results thus highlight the need to take into account the inhomogeneity of CNT-filled resins for accurate evaluation of the residual stress.