Abstract: To investigate the cooling effect of HP steam on IP rotor in huge power steam turbine unit, flow and heat transfer simulations on the first two stages of IP cylinder in a 600 MW steam turbine were performed using CFD and conjugate heat transfer (CHT) methods. Cooling effects of HP cooling steam on IP rotor disc and root as well as the influences of cooling steam flow behavior near stage hub region were investigated. Results show that with the cost of stage efficiency decrease, HP cooling steam can effectively cool the IP rotor disc so as to decrease significantly the rotor disc maximum temperature of IP first two stages.