This paper presents the design and development of ECU simulator in order to verify automatic firmware update system of ECU SW on the vehicle. We designed and developed general-proposed ECU simulator as devide into HW and SW parts. HWadopt 32bit MPU, CAN, LIN, LCD, and touch-pad button to satisfy general-proposed ECU level. And SW constructed as finite-state-machine structure in order to support HW spec. We developed ECU manager which deliveries update data to verify validity of operation. And we tested ECU simulator with ECU manager. For test of validity of ECU simulator, we set 4 scenarios between ECU simulator and manager. As the result, we confirmed validity of operation in ECU simulator for automatic SW update system.