A method and apparatus for indicating sidelink (SL) radio link failure (RLF) in a wireless communication system is provided. A first wireless device may initialize the counter to zero upon 1) establishing the PC5-RRC connection with the second wireless device, or 2) configuring or reconfiguring the maximum number of the counter. A first wireless device may increase the counter based on no reception of any acknowledgement to the transmission of the MAC PDU. A first wireless device may indicate Sidelink (SL) Radio Link Failure (RLF) for the PC5-RRC connection based on that the counter reaches the maximum number of the counter.