Heat input during welding can compromise the final quality of a weld and also add to the shortening life span of the components. One of the major effect of excessive heat input on weldments is microstructure alteration of the weld area. Therefore to obtain a quality weld, one need to take into cognizance the amount of heat being introduced into a weld zone. This research is aimed to optimize and predict heat input needed for fusion welding of mild steel plate. Response Surface Methodology (RSM) using the central composite design was employed to analyze results gotten from the design of experiment, where twenty sets of experiments were carried out with weld samples measuring 60x40x10mm cut, welded and analyzed. At the end, RSM model was employed to optimize and predict the heat input of mild steel weldments. The model produced a numerical optimal solution of: current 120.00 Amp, voltage of 23.79 volt and a gas flow rate of 15.71 L/min resulting in a welded material having a heat input of 2.49985x106 J/m. the microstructural alteration produced by this heat input value obtained from this research is very minimal.