A data unit sending or receiving method and a related apparatus that belong to the field of communication technologies are provided. In this solution, a transmit end sends a first aggregated media access control protocol data unit (A-MPDU) and determines an MPDU that needs to be retransmitted in the first A-MPDU. The MPDU that needs to be retransmitted is included in a first A-MPDU subframe in the first A-MPDU. The first A-MPDU subframe includes a first MPDU delimiter, and the first MPDU delimiter includes a first end of frame (EOF) field and a first MPDU length field. The transmit end sends a second A-MPDU, where the second A-MPDU includes a second A-MPDU subframe, the second A-MPDU subframe includes the MPDU that needs to be retransmitted and a second MPDU delimiter, and the second MPDU delimiter includes a second EOF field and a second MPDU length field.