A digital broadcasting system and a method of processing data are disclosed. A receiving system of the digital broadcasting system may include a signal receiving unit, a demodulating unit, a demultiplexer, and an audio/video decoder. The signal receiving unit receives a broadcast signal including main service data and an RS frame including a plurality of MPH service data packets. The demodulating unit demodulates data of the RS frame. The demultiplexer identifies an MPH service data packet including an IP datagram of mobile service data with reference to an MPH header of each MPH service data packet in the RS frame, and when a stuffing data is inserted in the identified payload of MPH service data packet, removes the stuffing data from the payload and separates an audio and video data from IP datagram of the mobile service data of the payload, and outputs the separated audio and video data.