A method for transmitting a physical layer protocol data unit (PPDU) in a transmission opportunity (TXOP) and a device using the same are provided. The device transmits a request to send (RTS) frame to a plurality of receiving stations. The RTS frame includes a bandwidth field and a plurality of allocation fields. The bandwidth field indicates a first bandwidth in which the RTS frame is transmitted. Each allocation field indicates a bandwidth in which a clear to send (CTS) frame is to be sent by a corresponding receiving station. The device determines a transmission bandwidth of a PPDU to be sent by comparing the first bandwidth with a second bandwidth which is a total bandwidth indicated by the plurality of allocation fields.