The ultimate tensile strength of individual single-walled carbon nanotubes (CNTs) is measured in a high-resolution transmission electron microscopy equipped with a conducting atomic force microscopy unit. The values are correlated to every tube's structural details. The strength of similar to 100 GPa, approaching the theoretical limit for defect-free CNTs, is documented in tubes without visible defects; for those with spatially separated stepwise pentagon-heptagon defects, determining the breaking sites, it reduces to 40-70 GPa.