This study investigates electronic transport in polycrystalline graphene, focusing on the role of grain boundaries. Graphene, a two-dimensional form of carbon, is a promising material for future electronics, but its polycrystalline nature, characterized by grain boundaries, can significantly affect its electronic properties. The research develops a theory of charge carrier transmission through grain boundaries composed of periodic dislocations, based on momentum conservation. It finds two distinct transport behaviors: high transparency or perfect reflection of charge carriers over large energy ranges. First-principles quantum transport calculations confirm these behaviors, revealing that grain boundaries can be engineered to control charge currents without introducing bulk band gaps. The study shows that grain boundaries in graphene can form periodic structures with typical periodicities of 1–5 nm, where crystal momentum conservation plays a crucial role in charge carrier transmission. The transport properties depend on the grain boundary structure, with two main classes: class Ia/Ib, which allow for high transmission, and class II, which exhibit a transport gap. Class II grain boundaries, characterized by a transport gap of approximately 1.38 eV per nanometer, perfectly reflect low-energy carriers over a large energy range. The research also discusses the effects of disorder on charge transport across class II grain boundaries, showing that moderate amounts of short-range defects lead to low conductance in the transport gap. The study highlights the potential of grain boundary engineering for practical graphene electronics, with applications in field-effect transistors and other devices. The findings suggest that by controlling grain boundary structures, it is possible to achieve high on/off current ratios, making graphene a viable candidate for nanoscale electronic devices.This study investigates electronic transport in polycrystalline graphene, focusing on the role of grain boundaries. Graphene, a two-dimensional form of carbon, is a promising material for future electronics, but its polycrystalline nature, characterized by grain boundaries, can significantly affect its electronic properties. The research develops a theory of charge carrier transmission through grain boundaries composed of periodic dislocations, based on momentum conservation. It finds two distinct transport behaviors: high transparency or perfect reflection of charge carriers over large energy ranges. First-principles quantum transport calculations confirm these behaviors, revealing that grain boundaries can be engineered to control charge currents without introducing bulk band gaps. The study shows that grain boundaries in graphene can form periodic structures with typical periodicities of 1–5 nm, where crystal momentum conservation plays a crucial role in charge carrier transmission. The transport properties depend on the grain boundary structure, with two main classes: class Ia/Ib, which allow for high transmission, and class II, which exhibit a transport gap. Class II grain boundaries, characterized by a transport gap of approximately 1.38 eV per nanometer, perfectly reflect low-energy carriers over a large energy range. The research also discusses the effects of disorder on charge transport across class II grain boundaries, showing that moderate amounts of short-range defects lead to low conductance in the transport gap. The study highlights the potential of grain boundary engineering for practical graphene electronics, with applications in field-effect transistors and other devices. The findings suggest that by controlling grain boundary structures, it is possible to achieve high on/off current ratios, making graphene a viable candidate for nanoscale electronic devices.