This article investigates the effect of grain structure on electromigration (EM) reliability of dual-damascene Cu interconnects with a CoWP capping layer, including the lifetime and statistics. Downstream EM tests were performed on two sets of CoWP-capped Cu interconnects with different grain sizes. Compared to Cu interconnects with the standard SiCN cap layer, the CoWP capping clearly improved the EM lifetime by ∼24× for the small grain structure and by another ∼14× for the large grain structure. Here, the effect of grain structure on EM lifetime was attributed to the grain boundary contribution to mass transport. The lifetime improvement, however, was accompanied with an increase in the statistical deviation, increasing from 0.27 for the SiCN cap to 0.53 for the small grain structure and to 0.88 for the large grain structure with the CoWP cap. This was attributed to the effect of grain structure in changing the statistical distribution of flux divergence sites and thus the failure statistics.