A method for Procedure Transaction Identity (PTI) error handling on 5G Session Management (5GSM) procedures is proposed. If a User Equipment (UE) receives a PDU SESSION AUTHENTICATION COMMAND or PDU SESSION AUTHENTICATION RESULT message in which the PTI value is an assigned value, the UE transmits a 5GSM STATUS message including 5GSM cause #81 “invalid PTI value”. If the UE receives a PDU SESSION ESTABLISHMENT ACCEPT or REJECT message, a PDU SESSION MODIFICATION COMMAND or REJECT message, or a PDU SESSION RELEASE COMMAND or REJECT message in which the PTI value is an unassigned value, the UE ignores the received message. If the UE receives any 5GSM message in which the PTI value is a reserved value, the UE ignores the received 5GSM message.