This study investigates the atomic structure and properties of grain boundaries in single-layer graphene, a hexagonal two-dimensional carbon crystal. The authors use a combination of transmission electron microscopy (TEM) techniques, including atomic-resolution imaging and diffraction-filtered imaging, to explore the grain structure and its impact on the mechanical and electrical properties of graphene. They find that graphene grains are predominantly connected via pentagon-heptagon pairs, and the grain boundaries are composed of carbon atoms and are stable under the electron beam conditions. The grain boundaries are shown to significantly weaken the mechanical strength of graphene membranes but do not significantly affect their electrical properties. The study also reveals that grain boundaries are decorated with surface particles and adsorbates, which may enhance their chemical reactivity. The techniques developed in this work provide a new tool for studying the structure and properties of grains and grain boundaries in graphene and other two-dimensional materials.This study investigates the atomic structure and properties of grain boundaries in single-layer graphene, a hexagonal two-dimensional carbon crystal. The authors use a combination of transmission electron microscopy (TEM) techniques, including atomic-resolution imaging and diffraction-filtered imaging, to explore the grain structure and its impact on the mechanical and electrical properties of graphene. They find that graphene grains are predominantly connected via pentagon-heptagon pairs, and the grain boundaries are composed of carbon atoms and are stable under the electron beam conditions. The grain boundaries are shown to significantly weaken the mechanical strength of graphene membranes but do not significantly affect their electrical properties. The study also reveals that grain boundaries are decorated with surface particles and adsorbates, which may enhance their chemical reactivity. The techniques developed in this work provide a new tool for studying the structure and properties of grains and grain boundaries in graphene and other two-dimensional materials.