The use of thick steel components that can be joined by one-layer-one-pass laser welding or laser-arc hybrid welding has been increasing with the trend of reducing the plate thickness by using higher strength steels. This has enhanced the demand for laser-arc hybrid welding of HT780 steels in the bridge and industrial machinery applications. Taking this trend into account, the authors have researched the suitable welding parameters and consumables for the hybrid welding process that couples laser beam and MAG arc for joining HT780 steel plates. As a result the following welding parameters have been proven to be most appropriate for better root-gap tolerance and lower spatter: MAG arc for leading (kept at the right angle)/laser for trailing (kept at a push angle); laser-arc distance of 3–5 mm; laser beam focus position of ± 0 mm; and shielding gas of Ar-20%CO2. As to the welding wire for laser-arc hybrid welding of HT780 steel, the HT590-class MAG arc welding wire of Cr-Mo-Ti type with a small amount of titanium has been revealed to be suitable to obtain sufficient weld metal tensile strength(890 MPa with 6-mm thickness; 925 MPa with 9-mm thickness; 919 MPa with 12-mm thickness) and toughness(VE−20=38 J with 5-mm subsize; VE−20=59 J with 7.5-mm subsize; VE−20=103 J with 10-mm full size).