Flexible printed circuits (FPCs) are widely used in electronic equipment such as mobile devices and wearable sensors. The conductive electric lines in these circuits are printed using nanoparticle metal ink and ink-jet direct write methods. Physical characteristics such as flexibility and mechanical durability of metal nanoparticle ink lines have been evaluated by bending or tensile tests. In contrast, the electrical characteristics of these lines have not been sufficiently evaluated, and the failure mechanism under high-density current has not been clarified. When electric devices are scaled down, current density and Joule heating increase in conductive lines and electromigration (EM) damage becomes a severe problem. Therefore, reducing the EM damage is extremely important to enhance the device reliability. In this study, a failure analysis of Ag nanoparticle ink lines were assessed using current loading tests and microscopic observations to discuss the damage mechanism and evaluate electrical reliability under high-density current. Atomic transport due to EM was observed at 60 kA/cm2 current loading, and relatively large aggregates and grain growth were observed at 120 kA/cm2 current loading. The time to open circuit was longer at 120 kA/cm2 than at 60 kA/cm2. The formation of large aggregates and unstable changes in the potential drop were observed at the two values of current density. It is considered that aggregate formation and grain growth affected the atomic transport by EM.