Recently, an analytical model to predict transient nugget growth behavior with weld cycle is developed in resistance spot welding process. In this thesis, a suboptimal controller is designed based upon the model, focusing on the regulation of nugget geometry and the minimization of control heat input needed for welding. The electric conductance is introduced to detect the nugget size and the correlation between the conductance and the nugget size is determined by experiments. Based upon these developments, the performance of the suboptimal controller is investigated by simulation study and experimental verification of the simulation results is performed by physical implementation of the controller to the practical spot welder using 16 bit microcomputer.