Abstract: The heat transfer in laser deep penetration welding includes heat conduction and convection in the welding pool.A 3D heat transfer model in laser deep penetration welding was developed. The main characteristics of the model has some characteristics as follows:(1)A prototype of the keyhole in the model was obtained from the experiments.(2)The convection and the temperature distribution in the depth were considered.(3)The same law in the solid and the liquid region, i.e.the viscosity of the material in the solid region was infinite;however the viscosity of the material in the liquid region was set to actual value.The model was solved by a finite element method, and the results show that:The isotherms in laser welding are shaped as ellipses.The temperature gradient and the velocity gradient on the front keyhole wall are greater than those on the rear keyhole wall.The convection flow in the welding pool is driven by the surface tension.