A transformation-induced plasticity steel was welded by gas tungsten arc welding. The microstructure of fusion zone was analyzed by means of optical microscopy and scanning electron microscopy with EDS. It is found that fusion zone may be classified into two zones, the completely melted zone and the partially melted zone. The microstructure of completely melted zone consists predominantly of martensite and bainite, and that of partially melted zone consists mainly of martensite, bainite and ferrite. The formation mechanism of fusion zone microstructure is analyzed. The micro-hardness distribution of the joint was measured by microhardness tester. Test results show that the partially melted zone is softened, which is resulted from the formation of 20.6% ferrite. During the bending test, crack occurred at 125 degree bending angle. It is found that the crack originates from the partially melted zone because of deformation concentration.