The present invention provides a data transmission method and apparatus, where the method includes: performing, by UE, security processing on a NAS PDU by using a security parameter and a security algorithm, where data to be transmitted is encapsulated in the NAS PDU; and sending, by the UE, the NAS PDU to a serving SGSN by using an RNC. The present invention can improve efficiency of transmitting a small data packet, and further provide a security mechanism to ensure security of transmitting the small data packet.