We report theoretical investigations of quantum transport in monolayer transition metal dichalcogenide (TMDC) tunneling field effect transistors (TFETs). Due to the specific electronic structure of TMDC WTe(2), a transmission valley is found in the conduction band (CB). For a proper choice of the doping, gate and supply voltages the WTe(2) TFET can produce a giant negative differential resistance (NDR) with a peak to valley ratio as large as 10(3). The mechanism of NDR is identified to be due to a transport-mode bottleneck, i.e., the band to band tunneling from the valence band of the source is partially blocked by a transmission valley of the CB of the drain. More generally, our calculations show that electronic structures of at least six TMDC materials possess the transmission valley.