A method to determine the modal S-parameters of the input / output ports of the equipment under test (EUT) connection of the conducted disturbance measurement system is investigated. Since the structure of the input terminal of the EUT connection is AC power plug itself, the modal S-parameter at the input port of the EUT connection cannot be measured directly with a vector network analyzer (VNA). To measure the modal S-parameter, the EUT connection needs to be measured through the AC-SMA adapter. In order to remove the effect of the AC-SMA adapter and estimate the unknown modal S-parameter of the EUT connection, the EUT connection with AC-SMA adapter is terminated with open, short and 50 Ω matching. From these measurement results, the modal S-parameters of input / output ports of the EUT connection are estimated. As a result, the modal S-parameters of the input / output ports are quite different. Therefore, the influence of the modal S-parameter due to the difference in the structure of the input / output ports of the EUT connection cannot be ignored.