Sumnrary: This paper describes the development of a three-dimensional numerical model for GTA (TIG) girth welding of pipes. It consists of a heat conduction model and molten pool balance model.In the heat conduction model, the transient temperature distribution and molten pool sizearenumericallyanalysed by the finite difference and enthalpy methods to consider the effect of the latent heat of fusion. In the molten pool model, the molten pool size derived from the heat conduction model is used to calculate the transient theoretical configuration of the molten pool, taking into account the balance ofgravity and surface tension.This model is classified as the rotational pipe and fixed pipe model.The effects of the material properties and welding parameters on the girth welding process are discussed on the basis of the numerical analysis. The results show that the molten pool in fixed pipe welding is more sensitive to burnthrough than the molten pool in rotational pipe welding.