This paper presents the operational performance and transient response of a high temperature gas-cooled reactor (HTGR) with an emphasis on the gas turbine through a two-dimensional approach. For its operational and transient simulation we use a GAMMA-T in which the system code, GAMMA, is coupled with the two-dimensional turbomachinery model. We also implement several models into the GAMMA-T: the reactor kinetics model, the bypass valve model, and the models of the core, the heat exchangers, the gas turbine, and the piping. The estimations of compressor and turbine performances are based on a two-dimensional axisymmetric throughflow method that is capable of predicting both the transient and steady-state behavior of the power conversion system (PCS). To demonstrate the code capability, we investigated the two representative transients of GTHTR300, which is a 600 MW direct cycle helium cooled reactor consisting of a prismatic block type core, a horizontal single-shaft configuration of turbomachinery, a recuperator, and a precooler: a loss of heat rejection transient corresponding to the failure of the precooler water supply, and a 30% load reduction transient from nominal operation with bypass control. The simulation results demonstrated the controllability and operational stability for the plant. (C) 2009 Elsevier B.V. All rights reserved.