In this paper, stability and internal dynamics for a gas metal arc welding (GMAW) process will be studied. GMAW process is considered as a nonlinear MIMO system and input-output feedback linearization method will be applied for control purposes. Internal dynamics is the unobservable part of the system dynamics; its stability analysis is a vital step in the investigation of the system stability as a whole. Also, drop detachment dynamics, will be considered here by tracking a saw-toothed arc length voltage Simulations results are presented to illustrate the system performance.