Linear instability of electromigration induced mass transport is studied as an intrinsic mechanism of void nucleation and growth in interconnect lines. Heat conduction along the conductor line and between the conductor line and surrounding materials is added to a recent model, proposed by Korhonen.et al. (1993) for electromigration induced stress evolution, to examine the role of thermomigration in linear instability of uniform mass transport. The analysis shows that thermomigration is the leading driving force for instability of electromigration-induced mass transport in interconnect lines. This theoretical prediction seems to qualitatively agree with a recent experimental study. These results suggest that thermomigration, which has been ignored in most previous studies, could play a significant role in electromigration failure of interconnect lines.