This paper presents a transient thermal analysis using Finite Element Method (FEM) to model the heat input effect of the Gas Metal Arc Welding (GMAW) process in plates of Al- 6 061-T6 with 4.8 mm of thickness. To validate the numerical results, a set of experiments were developed measuring temperatures along a perpendicular line from the weld cord. A good agreement was found between numerical and experimental results. This model could be use as a tool to analyze microstructural changes on the material.