A theoretical model of nanovoid nucleation at triple junctions in nanocrystalline materials is developed in this article. The sliding of grain boundaries (GBs) meeting at triple junctions, which can be attributed to the gliding of GB dislocations (GBDs), provides the driving force for nanovoid nucleation. The GB sliding is accommodated by the emission of partial dislocations from GBs as well as GB diffusion. The corresponding energy characteristics of the pile-ups of GBDs, the emission of partial dislocations from the GBs, and GB diffusion are calculated, respectively. Furthermore, an energy balance method to calculate the nucleation of nanovoid at triple junctions is studied. The analysis demonstrates that the nucleation of the triple junction nanovoid depends mainly on the applied stress, the GB length (length of the pile-up), the GB structures, and the GB sliding accommodations.