In this paper, a real time simulator of the engine-generator for a series hybrid electric bus is designed which can be used for hardware in the loop testing of the hybrid bus control unit. As an important step for designing a hardware in the loop simulator, messages that the engine and generator receive from the vehicle control unit are identified. Design of the simulator is based on these received messages and dynamic behavior of the components. Since the engine and generator receive speed or torque commands from the vehicle control unit, two PID controllers are designed to bring the engine or generator to the desired speed. By applying appropriate inputs, different modes of operation of the diesel-generator is simulated including the soft start process and steady state operation. For a simulator, the ability to interface with a real hardware requires a real time simulation. To do so, the design process is implemented in LabVIEW environment and results show that the designed simulator gives responses close to real test results and it is suitable for developing the hybrid vehicle control unit.