A method for indicating a frame structure in a wireless local area network (WLAN) is provided. A sending device generates a transmission frame. The transmission frame includes a physical header. The physical header includes a frame structure indication. A subcarrier polarity of the frame structure indication is used to indicate a frame structure of the transmission frame. The sending device sends the transmission frame to a receiving device. In the transmission frame, a first subcarrier polarity indicates the transmission frame has a first frame structure, in which the physical header includes a field and a time domain repetition of the field; and a second subcarrier polarity indicates the transmission frame has a second frame structure, in which the physical header includes the field but does not include the time domain repetition of the field. The frame structure indication is an orthogonal frequency division multiplexing (OFDM) symbol.