In this work, the thermal behavior of ink-jet-printed nanoparticle ink during electrical sintering was demonstrated. The ink consisting of silver nanoparticles approximately 50 nm in size and 34 wt% was used. Constant currents of 0.11, 0.22, and 0.31 A were applied to Joule-heat the inkjet-printed silver nanoparticles. During the sintering process, in-situ voltage and current measurements were taken to calculate the heat source and thermal conductivity. In order to estimate the temperature during the electrical sintering process, numerical modeling of the two-dimensional heat conduction equation was adopted. Thermal conductivity was obtained from the in-situ electrical conductivity measurement and coupled to the numerical model using the Wiedemann Franz law. From these numerical modeling results, the relationship between the specific resistance of the ink and the temperature was determined. During the electrical sintering process, the specific resistance of the ink was strongly related to the sintering temperature. The specific resistance of the ink decreases as the process temperature rises.