The paper presents a thermo-economic analysis of gas/steam combined cycle. The stated objective is achieved by optimizing thermo-economic parameters for simple combined cycle (large and medium range) and to apply this to economic model of these cycles. The economic parameters evaluated in the present study include discount cash flow rate of return (DCRR) and gross payout period (GPO), two terms commonly employed in engineering economic analysis. DCRR and GPO are calculated for various electric sale and fuel prices. It has been found that maximum value of DCRR and minimum value of GPO are found with large size plant.