This paper describes on/off design performance of a 50KW turbogenerator gas turbine engine for hybrid vehicle application. For optimum design point selection, a relevant pa4rameter study is carried out. The turbogenerator gas turbine engine for a hybrid vehicle is expected to be designed for maximum fuel economy, ultra low emissions, and very low cost. A compressor, combustor, turbine, and a permanent-magnet generator will be mounted on a single high speed (80,000 rpm) shaft that will be supported on air bearings. As the generator is built into the shaft, gearbox and other moving parts become unnecessary and thus will increase the system’s reliability and reduce the manufacturing cost. The engine has a radial compressor and turbine with design point pressure ratio of 4.0. This pressure ratio was set based on calculation of specific fuel consumption and specific power variation with pressure ratio. For the turbine inlet temperature, a rather conservative value of 1100K was selected. Designed mass flow rate was 0.5 kg/sec. Parametric study of the cycle indicates that specific work and efficiency increase at a given pressure ratio and turbine inlet temperature. Off design analysis shows that the gas turbine system reaches self operating condition at about N/NDP = 0.48. Bleeding air for a turbine stator cooling is omitted considering the low TIT in the present engine and to enable the simple geometric configuration for manufacturing purpose. Various engine performance simulations including ambient temperature influence, surging at part load condition; transient analysis were performed to secure the optimum engine operating characteristics. Surge margin throughout the performance analysis were maintained to be over 50% approximately. Present analysis will be compared with performance test result which is scheduled at the end of 1998.