A digital broadcast system and a data processing method are disclosed. A data processing method of a digital broadcast transmission system includes delaying a reference time of a program clock reference (PCR) based on a size of mobile service data, when processing a broadcasting signal including main service data and the mobile service data, verifying a transport stream system target decoder (T-STD) model based on the PCR of the delayed reference time, and storing a packet of the main service data in an auxiliary buffer, when overflow of a buffer in the T-STD model is estimated as the verification result of the T-STD model.