A method for efficiently setting an external device to execute an interworking function in an in-vehicle multimedia system and a method for performing the same are provided. The method includes connecting at least one external device to the in-vehicle multimedia system and then detecting a number of connected external devices. Upon detecting that the number of connected external devices is one, the one external device is set as a default device to execute the interworking function. Upon detecting judging that the number of connected external devices is multiple, a pop-up window is displayed to select one of the plurality of external devices on a display, and the external device selected through the pop-up window is set as the default device.