This application provides a transmission method and apparatus. The method includes: A first terminal device obtains information about N SL transmission units. The N SL transmission units are used on an SL for sending a data packet of a unicast connection or a groupcast communication group. The first terminal device transmits the data packet to a second terminal device by using an unused SL transmission unit in the N SL transmission units through a HARQ process used for the unicast connection or the groupcast communication group. The second terminal device sends a HARQ feedback result to the first terminal device based on a decoding result. When the first terminal device has not received the HARQ feedback result or the received HARQ feedback result indicates that the data packet fails to be transmitted, the first terminal device retransmits the data packet by using a next unused SL transmission unit in the N SL transmission units. A HARQ feedback retransmission mechanism and a corresponding resource allocation method are introduced into SL transmission, to improve reliability of a data packet in the SL transmission.