To make progress of laser-arc hybrid welding, especially which is applied to thick steel plate, it is essential to get remarkable synergic effect of deep penetration. It is suggested that the effective process is combined hollow cathode TIG with high beam quality laser. Penetration of hollow TIG/YAG hybrid welding was investigated by numerical analysis. A keyhole surface model taken account of laser beam parameter product (BPP) was developed. The calculated keyhole surface profiles of coaxial hybrid welding are wider and deeper than that of YAG laser alone. As a result, 1-pass butt welding of 20mm thickness steel plate was achieved in condition of BPP=0.9mm · mrad at laser power 1.8kW. To confirm numerical results, coaxial hybrid welding tests with hollow cathode TIG was conducted. The welding penetration was about 2 times as deep as YAG laser alone. It was found that coaxial hybrid welding of hollow TIG/YAG had large synergic effect.