A method and apparatus for discarding data among associated transmission buffers in a wireless communication system is provided. A wireless device generates a first data packet and a second data packet, and performs a random access (RA) procedure based on the first data packet. When the RA procedure based on the first data packet fails, the wireless device discards the first data packet, and continues the RA procedure based on the second data packet. A media access control (MAC) layer indicates a radio resource control (RRC) layer that the RA procedure based on the second data packet is performed.