A loop type pressure-gradient-driven low-pressure thermosiphon device includes a case sealed by a cover to define a chamber with a vaporizing section. The vaporizing section includes a plurality of spaced flow-guiding members and first flow passages formed between adjacent flow-guiding members. The flow passages respectively have at least one free end communicating with a free zone in the chamber. A pipeline is connected at two ends to two opposite sides of the case, and has a second flow passage communicable with the vaporizing section. The pipeline extends through at least one heat-dissipating element, so that the pipeline and the heat-dissipating element together define a condensing section. In the thermosiphon device, a low-pressure end is created through proper pressure-reduction design to form a pressure gradient for driving steam-water circulation, and the working fluid can be driven to circulate and transfer heat in the pipeline and the case without any wick structure.