Abstract: Disclosed are a device for transmitting a data unit and a method of operating the same. More particularly, the device of the present disclosure includes a size determination unit for determining an optimal split size for a MAC Service Data Unit (MSDU) received from an upper layer by applying a transmission time algorithm; a unit division unit for splitting the MSDU into the determined size; and a MAC layer management unit for generating plural MAC Protocol Data Units (MPDUs) based on the split plural MSDUs and the delimiter for each of the split plural MSDUs, generating an aggregate protocol data unit by applying an aggregate transmission scheme to the generated MPDUs, and delivering the generated aggregate protocol data unit to a physical layer, thus guaranteeing reliability important for video streaming and, at the same time, increasing the throughput.