Various solutions for single network slice selection assistance information (S-NSSAI) based congestion control with respect to user equipment and network apparatus in mobile communications are described. An apparatus may receive a message with a back-off timer from the network node. The apparatus may determine whether an S-NSSAI of a protocol data unit (PDU) session is provided by the network node. The apparatus may start the back-off timer and associate the back-off timer with the S-NSSAI of the PDU session in an event that the S-NSSAI is provided by the network node.