An organic rankine cycle (ORC) turbo generator for prevention of penetration of a working fluid is provided, which can block penetration of the working fluid from a turbine into a generator, thereby improving the efficiency of power generation. The turbo generator comprises: a turbine including a turbine housing, a turbine blade and a fluid injection nozzle; a generator including a generator housing connected to the turbine housing, a rotor rotatably mounted inside the generator housing and a rotary shaft mounted to the turbine blade, and a first bearing and a second bearing mounted at the front and rear ends of the rotor; and working fluid penetration-preventing means adapted to block the pressure generated from the turbine so as to prevent the working fluid injected from the fluid injection nozzle from being penetrated into the generator.