Abstract: A three-dimensional mathematical model for the temperature field and flow field in full penetration laser welding of titanium alloy was presented. In this model, the heat source comprises a plane heat source on the top surface and a cylindrical heat source along the z-direction, which takes into account the plasma effect and the keyhole absorption. The mushy region is introduced to provide a simple method to dispose of the pressure and velocity boundary conditions. Calculated results from the models are found to agree with the experimental results for the geometry profile of weld. The metal flow is the main cause for forming the typical "hourglass" cross-section profile.