This paper presents a static and dynamic simulation model of a Super Critical Heat Recovery Steam Generator (SC HRSG) and its application to investigate the load changes and start-up processes for the next generation high efficiency combined cycles. To design the modem combined cycle power plants with high pressure and temperature steam requirements, it is necessary to generate detailed computer models. As an important supports for designers this study describes the upgrading of a validated subcritical HRSG model to SC HRSG. The model includes advanced control circuits to achieve a high level of accuracy, especially during startup. The comparison between the simulation results and the designed data in steady state and different load changes are documented. The obtained results demonstrate that the simulation is very reliable to predict the startup procedure for the SC HRSG. And differences in the thermal efficiency between Sub & SC HRSGs during warm start-up process are assessed.