A data transmission method includes: receiving, by a receive end, a Radio Link Control protocol data unit (RLC PDU); determining, by the receive end, whether the RLC PDU includes a complete service data unit (SDU) or an SDU segment, where the RLC PDU is transmitted in unacknowledged mode (UM); and transmitting, by the receive end to a Packet Data Convergence Protocol (PDCP) layer based on a result of the determining, the complete SDU or a complete SDU assembled by using the SDU segment. The data transmission method implements data transmission in UM at an RLC layer in 5G.