In recent years, hybrid laser-arc welding has been developed and applied in many applications because of its advantages over laser welding and arc welding alone, which is mainly due to the synergic effect of laser and arc. Although some progress has been made in the development of hybrid laser-arc welding technique, lack of understanding of some fundamental principles in this emerging joining technique has hindered its further development and advancement. For example, in hybrid laser welding, one way to prevent/eliminate cracking is to add filler metal with anti-cracking element and diffuse them well into the weld pool in welding. Thus, the mass loss can be compensated and compositional and mechanical properties of the welds can be improved. Understanding the mass transfer in molten pool is vital to achieve these desired objectives. In this paper, melt flow and mass transfer in hybrid laser-GMA keyhole welding will be investigated. Mathematical models will be developed to investigate melt flow, energy transport, and mass transfer in hybrid laser-GMA keyhole welding. The effects of welding parameters like feed wire diameter and welding current on melt flow and mass transfer in weld pool will be discussed. Copyright © 2013 by ASME.