Embodiments of the present disclosure relate to a method and an apparatus for allocating a PDU session ID for a terminal device. The method comprises: selecting (S101), for a terminal device, a protocol data unit session identity, PDU session ID, from a group of PDU session IDs. The group of PDU session IDs are not allocated to the terminal device. According to the embodiments of the present disclosure, the network node may select, for the terminal device, a PDU session ID from a group of PDU session IDs, which are not allocated to the terminal device yet. Thus, any newly selected PDU session ID may be ensured as unique.