Abstract: The influence of applied external magnetic field generated by a pair of bar permanent magnets on the microstructure and mechanical property of flash butt welded mild steel joint was analyzed.Cast structure and lots of oxide inclusions are found in the weld line without external magnetic field,and the heat affected zone(HAZ) is wider compared to the joint with magnetic field.Instead of cast structure,there are equiaxed ferrite with a small amount of Widmanstatten structure and pearlite in the weld line with magnetic field,and the grain coarsening in the HAZ is not obvious.In the tensile tests,the samples without external magnetic field fail in the HAZ with the brittle facture,while the samples with magnetic field rapture in the base metal and obvious plastic deformation is found.The average tensile strength of the samples with magnetic field is higher compared with those without magnetic field.As a result,the joints by flash butt welding can be ameliorated through applying a magnetic field.