Printed electronics (PEs) have attracted attention for the fabrication of microscale electronic circuits. PEs use conductive inks which include metal nanoparticles. The conductive ink can be printed on flexible substrates for wearable devices using ink-jet printers and roll-to-roll methods. With the scaling down of electric devices, the current density and Joule heating in the device lines increase, and electromigration (EM) damage becomes significant. EM is a transportation phenomenon of metallic atoms caused by the electron wind under high-density current. Reducing the EM damage is extremely important to enhance the device reliability. With the progress in miniaturization of the metal nanoparticle ink lines, EM problem needs to be solved for ensuring the reliability of these lines. We know that the formation of aggregates and cathode damages occur due to a current loading. The diffusion path of atoms due to the EM has not been identified under the high-density current loading. In this study, a high-density electric current loading was applied to an Ag nanoparticle line. The line specimens were prepared using a lift-off method. After the current loading tests, observations were conducted using a laser microscope and scanning electron microscope. A local decrease in the line thickness and scale-shaped slit-like voids were observed due to the high-density current loading. Moreover, the microstructure of the line was modified by enlarging the Ag grain. From the results, we identified that a dominant diffusion occurred at the Ag grain boundary due to the EM.