Abstract: The effects of filler wire on the microstructure, mechanical properties and tensile failure mechanism of welded joints were studied. The results show that the average Al content in the weld seam is 1.90% without filler wire, and the microstructure contained martensite and coarse δ-ferrite. The tensile strength and elongation of the welded joints were 1 340 MPa and 1.80%, respectively. Due to the significant hardness difference (142 HV) between δ-ferrite and martensite, the crack originated from the phase interface between δ-ferrite and martensite. The average Al content in the weld seam was reduced to 0.96% with filler wire. Due to the dilution of Al by filler wire, the microstructure was full martensite and the tensile strength and elongation of the welded joint were increased to 1 510 MPa and 4.4%. The hardness of martensite in the weld seam (491 HV) was lower than that in the base metal (523 HV) because the filler wire also dilutes the C. During tensile test, the crack initiated and propagated in the martensite and finally filed in the weld seam.