A method of network resource release processing is provided, which includes the following steps. After user equipment (UE) using idle mode signaling reduction (ISR) mechanism registers to two 3rd Generation Partnership Project (3GPP) communication networks, when the UE changes from the 3GPP network to a non-3GPP communication network, a serving gateway (serving GW) receives a message sent from a peer endpoint network element (NE), and deletes network resources established by the two 3GPP communication networks for the UE according to the message. A mobility management NE and a serving GW are also provided. Through the method and device of network resource release processing, the resources are released when the UE using the ISR mechanism changes from the 3GPP network to the non-3GPP communication network.