To　investigate　the　fracture　properties　of　weathering　steel　Q355GNHB,　calibrate　its　mesoscopic　fracture　model　parameters,　and　realize　the　prediction　of　the　fracture　behavior　of　weathering　steel,　this　study　conducted　monotonic　tensile　tests　and　cyclic　loading　tests　on　54　notched　round　bar　specimens　from　the　base　metal　zone,　welded　metal　zone,　and　heat-affected　zone　(HAZ)　of　weathering　steel　Q355GNHB　at　room　temperature.　Utilizing　the　characteristic　length　values　obtained　from　the　electron　microscope　scanning　experiments　combined　with　the　void　growth　model　(VGM),　the　stress　modified　critical　strain　(SMCS),　and　the　cyclic　void　growth　model　(CVGM)　mesoscopic　fracture　mechanism　model,　we　calibrated　the　mesoscopic　fracture　toughness　parameters　of　the　materials　in　various　regions　of　Q355GNHB　weathering　steel.　Based　on　the　existing　results　of　the　smooth　round　bar　specimen　experiments,　the　accuracy　and　applicability　of　the　fracture　toughness　parameters　calibrated　by　these　three　models　were　verified.　The　analysis　results　indicate　that　the　material　in　the　base　metal　zone　exhibited　the　best　fracture　performance　during　the　monotonic　tensile　tests,　while　the　materials　in　the　other　two　areas　displayed　lower　fracture　toughness　parameters　due　to　the　effects　of　welding.　The　cyclic　loading　tests　revealed　that　materials　in　the　heat-affected　zone　are　more　prone　to　fatigue　fracture.　The　average　characteristic　length　value　of　the　materials　in　the　base　metal　zone　was　significantly　greater　than　those　in　the　welded　metal　zone　and　the　heat-affected　zone.　Both　the　VGM　and　SMCS　models　demonstrated　good　predictive　performance　for　crack　initiation　displacement　values　of　specimens　in　all　regions,　while　the　CVGM　model　exhibited　higher　prediction　accuracy　for　fracture　displacement　and　fracture　cycle　counts　of　specimens　in　all　regions.　The　mesoscopic　fracture　mechanism　model　is　applicable　to　weathering　steel,　accurately　describing　and　predicting　the　fracture　behavior　of　weathering　steel　with　good　precision,　thereby　providing　a　reference　basis　for　the　study　of　fracture　behavior　in　weathering　steel　structures.　©　2024　Elsevier　Ltd