This disclosure proposes a method to achieve D2D (device to device) communication in LTE system. Traditional communication in LTE systems is based on eNB. UEs (User Equipment) need to connect to eNB to send uplink and downlink transmission. Device to device communication is different from the traditional LTE scheme. UEs may directly connect to other UEs, sending data to each other. The disclosure provides the signaling process, based on LTE network entry procedure, to establish D2D communication in LTE system. D2D UEs send Msg1 to another D2D UE, and the recipient respond with Msg2. After the negotiation between D2D UEs, Msg3 and Msg4 are exchanged between UEs and eNB. Several embodiments are provided for the signaling of Msg3 and Msg4. The D2D data transmission may be enabled after the Msg4 or explicit data session initialization.