Abstract: The macrostructure of laser welding for titanium alloy was investigated in this paper. The results show that unstable full penetration will occur because of influence of laser induced plasma, even though laser welding parameters were stable. This phenomenon was characterized by perfect weld surface, and unstable weld back that part penetration and full penetration formed by turns. It is assumed that unstable full penetration was intrinsic for laser penetration welding and depended on significantly the drilling speed during the keyhole forming. According to the energy balance on keyhole wall, the mathematical model for the smallest laser power density that made the stable full penetration weld was suggested in this paper, which allowed laser power density to be related to material properties, sheet thickness the, drill speed and laser welding speed. The computed results were corresponded with the experimental results.