A method of enhanced error handling for 5G QoS operations is proposed. A PDU session defines the association between the UE and the data network that provides a PDU connectivity service. Each PDU session is identified by a PDU session ID, and may include multiple QoS flows and QoS rules. Within a PDU session, there should be one and only one default QoS rule. The default QoS rule indication (DQR) and the QoS flow identifier (QFI) of a signaled QoS rule should not be changed. Two new 5G session management (5GSM) causes are defined: a semantic error in the QoS operation (cause value #83) and a syntactic error (cause value #84) in the QoS operation. The UE should check the QoS rule provided in a PDU session modification command message for different types of QoS rule errors.