An optimum analysis method is proposed for a HTGR cogeneration system from the viewpoints of economy, energy and exergy based on the EXCEM concept (exergy, cost, energy and mass). In this method, first the whole HTGR cogeneration system is simulated based on the thermodynamic relations and energy and mass conservation law. Then, based on the design parameters and initial data, the exergy is used to analyze the performance of each component and decide the cost of each product in the cogeneration system. Lastly, the iterative optimum technique is employed to improve the objective function-maximum exergy efficiency. In this study, a software system-¡ÈEXCEM Studio¡É is developed to put this method into practice to solve abundant equations and represent the result graphically. The optimum results by using this method prove that it is feasible to optimize the design of the HTGR cogeneration system.