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A method is presented in this paper to design proportional-integral (PI) controller for industrial processes which are modeled by second order plus dead time transfer function. Frequency domain approach is employed in order to construct this method. For this purpose some frequency characteristics constant loci such as gain and phase margins constant loci are determined and plotted in the controller parameters plane. These loci and other performance indexes are used to determine the controller parameters. That is, the desired closed loop system characteristics are stated in the form of two loci, and drawing the loci, determining their intersection point coordinates (if exist), and then the controller parameters are calculated. An illustrative example is given to demonstrate the proposed method.