A method of detecting a data bit depth and an interface device for a display device using the same are disclosed. The method includes confirming a physical connection between a transmitting terminal and a receiving terminal and then transmitting a clock data recovery (CDR) training pattern signal from the transmitting terminal to the receiving terminal, outputting clocks from a CDR circuit of the receiving terminal using the CDR training pattern signal, receiving an alignment training pattern signal subsequent to the CDR training pattern signal from the transmitting terminal to the receiving terminal, and counting bits of pixel data included in the alignment training pattern signal or the clocks and determining a data bit depth of input data based on a count result, in the interface receiving terminal.