This invention relates to a debug device and method thereof and is applied to detect transmission on a bus in a computer system having a CPU, a north bridge chip and a south bridge chip. The debug device consists of a processing unit, a comparing unit and a recording unit. The processing unit receives a first address signal and a second address signal from the north bridge chip, and correspondingly transmits an index signal and a test data to the north bridge chip. The comparing unit compares the first and the second address signal to generate a comparing signal. And the recording unit records the first and the second address signal and the comparing signal. The north bridge chip connects to the south bridge chip via a bus, and the debug device also connects to the south bridge chip. Therefore the north bridge chip and the debug device transmit to both through the south chip.