To　better　predict　the　ductile　fracture　of　welded　joints　in　steel　bridges　and　buildings　under　seismic　load,　the　fracture　behavior　of　Q345　steel　that　has　been　widely　used　in　China　was　studied　under　both　monotonic　and　cyclic　loadings　based　on　the　microscopic　mechanism　of　material　fracture.　Based　on　the　results　of　notched　round　bar　tensile　tests　and　finite-element　analyses　for　Q345　steel　base　metal,　weld　metal　(transverse　and　longitudinal),　and　heat-affected　zone　specimens,　the　parameters　of　the　void　growth　model,　stress-modified　critical　strain　model,　and　cyclic　void　growth　model　were　calibrated.　The　parameters　of　characteristic　length　in　the　previous　models　were　determined　by　scanning　electron　microscope　tests.　The　results　indicated　that　the　toughness　parameters　of　the　four　materials　are　significantly　different,　and　the　cyclic　growth　capacity　of　the　voids　in　weld　metal　and　heat-affected　zones　deteriorates　severely　under　cyclic　loading.　The　toughness　parameters　of　the　two　oriented　weld　metals　are　similar;　therefore,　the　fracture　toughness　of　the　weld　metal　is　less　affected　by　the　material　orientation.　The　coefficients　of　variation　of　the　toughness　parameters　of　the　four　materials　were　small,　and　the　distributions　of　the　fracture　indexes　of　the　specimens　with　different　notch　radii　were　the　same,　which　proved　the　effectiveness　of　the　microscopic　mechanism　fracture　model　in　predicting　the　initiation　of　ductile　cracks　of　the　members　with　different　geometric　shapes　and　stress　states.　The　results　of　this　paper　provide　the　required　material　data　and　model　parameters　to　predict　the　ductile　fracture　of　welded　joints　made　of　Q345　steel　using　existing　micromechanical　fracture　models.