A data transmission device connected to a monitor includes a wireless transmission module, a multimedia processing unit and an output interface. The wireless transmission module wirelessly receives a multimedia data transmitted by an electronic apparatus. The multimedia processing unit receives and decompresses and decodes the multimedia data, and the output interface transmits the multimedia data to the monitor for displaying the multimedia data. The display area of the monitor is larger than the display area of the display unit of the electronic apparatus. A user is allowed to watch the multimedia data generated from the electronic apparatus executing a multimedia file on the monitor with a large display area.