A data transmission system has a master circuit, a slave circuit, and a transmission control circuit. The slave circuit stores a plurality of data in a first format. The master circuit processes data in a second format to perform a corresponding function. The transmission control circuit is coupled to the master circuit and the slave circuit. The transmission control circuit accesses a first datum from the slave circuit according to a first access command of the master circuit, converts the first datum in the first format into a first application datum in the second format, and transmits the first application datum to the master circuit.