The present study focuses on the morphology of the weld zone, especially on the quantitative microstructural characterization of bainitic structures, and the relationship between post-weld bainitic structures and the hardness at the weld zone of the friction-stir welded X80 API-grade pipe-line steel. The employed rotating and traverse speeds of the polycrystalline cubic boron nitride tool during the friction-stir welding process were 350 r/min and 12.7 cm/min (5 inch per minute), respectively. A microstructural analysis of the weld was carried out using optical microscopy, orientation imaging microscopy and transmission electron microscopy, and the hardness variation was also mapped across the weld-plate cross section. The frictional heat and plastic flow during friction-stir welding create a granular bainitic structure (the stir zone), a lath-like upper bainitic structure (the hard zone) and a recrystallized equiaxed polygonal ferritic structure (the heat-affected zone) in the weld zone. The maximum hardness was observed in the classical upper bainitic microstructure in the hard zone. The minimum hardness was found in the heat-affected zone. A good correlation between the post-weld bainitic structures and the hardness in the weld zone was obtained; the hardness increases almost linearly with the decreasing bainite-lath and packet sizes.