Abstract: A hair extension device is made easily adjustable and convenient to use by providing attached adjustment mechanisms which are inconspicuous and easy to hide. The hair extension device is generally formed as a headband, including a supporting base panel and a connection strand. The supporting base panel can accommodate extension hair in various patterns, and has adjustment mechanisms attached thereto. The size adjustment mechanisms are specifically configured to cooperate with the connection strand to effectively shorten the length of the headband structure and thereby adjust the size of the device.