A method of control Maximum Transmission Unit (MTU) reporting and discovery using AT commands is proposed. In communications networks, the MTU of a communication protocol of a layer is the size (in bytes or octets) of the largest protocol data unit that the layer can pass onwards. In an IP network, IP packets may be fragmented if the supported MTU size is smaller than the packet length. In accordance with one novel aspect, the packet data protocol (PDP) context of a packet data network (PDN) connection comprises MTU information. By introducing MTU information to the PDP contexts, TE can use AT commands to query MTU parameters from the network and thereby avoid fragmentation. TE can also use AT command to set MTU parameters and thereby control MTU discovery.