In laser keyhole welding of alloys, low melting-point elements, such as aluminum or magnesium, can be easily vaporized and lost from the weld region, leading to the changes of composition, mechanical properties, and the susceptibility of hard cracking and other defects. In hybrid laser-MIG keyhole welding, compositions of the base metal and filler metal are usually different, allowing some "anti-crack" elements to be added through the filler metal in anticipation of reducing or eliminating the aforementioned problems. Apparently, in order to achieve the desired objectives, it is a prerequisite to have good mixing between the base metal and filler metal in the molten pool. Hence, it is very important to know how well the mixing between the base metal and fill metal would be and what parameters are controlling the mixing phenomena. In this study, mathematical models and the associated numerical techniques have been developed to investigate the mixing process in both spot and 3-D moving hybrid laser-MIG keyhole welding. The dynamics of the weld pool fluid flow and the interactions between droplets and weld pool are calculated as a function of time. The effects of droplet size (wire diameter), droplet frequency (wire feed speed), welding speed, and the distance between the MIG and laser beam on weld pool mixing are studied. It is found that the competition between the rate of mixing and the rate of solidification determines the compositional homogeneity of the weld pool. Parameters that can influence either the rate of mixing or the rate of solidification or both are investigated and discussed.