Apparatus and methods are provided for sidelink relay channel establishment. In one novel aspect, the relay UE established sidelink relay channels upon receiving sidelink relay connection request. In one embodiment, the request comes from the base station for the in-network relay UE. In another embodiment, the request comes from a remote UE requesting the service for the out-of-network relay UE. The relay UE performs the traffic relay between the two end nodes of the sidelink replay path at the L2 or at L3. In another embodiment, a specific PC5 RRC message is used to initiate the relay channel establish procedure between the relay UE and the remote UE. In another embodiment, the network sends the configurations of the relay channel to the relay UE before the start of the procedure. The mapping between relay channel ID and the end-to-end RB index is maintained in the relay UE for bearer mapping.