A low alloy high strength steel (11CrNi3MnMoV) of 16 mm in thickness is well joined by laser-metal active gas (MAG) hybrid welding and the hardness, tensile and impact properties of weld seam and base metal are analyzed, respectively. Furthermore, the fracture mechanism is investigated through fractography. The results of the tensile test indicate that the strength of weld seam is about 817 MPa, which is 13% higher than that of base metal. The dimples in the fracture of weld seam are fine and uniform, whereas that of base metal are large, which shows that the tensile strength of weld seam is much higher, while the tenacity of base metal is much better. The results of the impact test indicate that the impact energy of base metal changed slightly, while that of weld seam decreases gradually as the testing temperature reduced. In addition, the tenacity-fragility transformation occurres in weld seam as the testing temperature decreases to -40 ℃. The fracture of weld seam exists both ductile and brittle rupture zone, while that of base metal only exists ductile rupture zone.