A criterion for the coalescence of grain boundary nanocracks, formed in nanocrystalline materials (NCMs) in the course of grain boundary sliding, into a catastrophic macrocrack is developed. Estimates of the NCM ultimate strength, which characterizes the formation of such a macrocrack, and its strain to failure, are made. It is shown that the ultimate strength of an NCM deformed through grain boundary sliding considerably increases with an increase in the misorientation angles of NCM grains.