Abstract: In this study, stability of Gas Metal Arc Welding (GMAW) process and its internal dynamics 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. To investigate the stability of the internal dynamics, an intrinsic property of the nonlinear system is defined by considering the system`s internal dynamics when the control inputs are such that the outputs are maintained at zero which is called zero dynamics. Details of zero dynamics matter because if unstable, it can invalidate many control design procedures such as input-output feedback linearization method. At the present study, the stability of zero dynamics of the process which was ignored in the previous works will be investigated. Also, the state space model of the GMAW process will be modified by considering mass rate of the vaporizing electrode, which has a critical influence on the stability of the internal dynamics. Furthermore an input-output feedback linearization method will be used to design a controller for regulation of the arc length and current of the process and simulations results will be presented to illustrate the controller performance.