An electronic toll collection (ETC) system and an on board unit (OBU) thereof are provided. The ETC system includes a toll collection module and an identification module. The toll collection module provides a pre-pay service and a post-pay service such that the OBU on a vehicle can make a toll payment in response to one of the pre-pay service and the post-pay service. The identification module captures an image of the vehicle and provides an identification service to the OBU to obtain an identification information of the vehicle.