Abstract: The Rehai (Hot Sea) geothermal field at Tengchong County of Yunnan Province is the largest one in mainland China. Based on the results of geological, geochemical and geophysical reconnaissance surveys since 1973, a conceptual model of the Rehai field can be built. Volcanism started in Tengchong in the Upper Miocene/Pliocene, reached a peak about 1 Ma ago, and lasted during the whole Pleistocene. The effect of this long period of volcanic activity was to produce the Rehai geothermal field. The surface manifestations of the Rehai field include boiling springs, fumaroles and steaming ground, with temperature of 96℃. The springs discharge sodium-chloride-bicarbonate waters with TDS of 2g/kg. Based on micro-earthquake and magneto-telluric studies, it is inferred that a magmatic heat occurs beneath the Rehai field. It appears to be a cooling intrusion below about 6-7 km depth. The anoma1ously high 3He/4He ratio of the Rehai field confirms the existence of degassing upper mantle rocks at crustal 1evels. The texture of the thermal field has been surveyed by reflection seismic method. The cap rock of the Rehai field is composed of Miocene stratium of about 300m thick, and the reservoir rock is the low velocity layer within the Precamberian Gaoligongshan group about 1500m below surface. According to some temperature data in shallow drillholes, the thermal gradient is as high as to 100℃/lOOm. Because of the lack of deep drillhole in the field, reservoir temperature in the range of 230℃ to 275℃ can be estimated by cation geothermometers. The temperature at top of cooling magma pocket is estimated to be about 670℃ by higher conductive heat flow of 288 mW/m2 measured in the low-permeability Miocene rocks at Liuhuangtang-huangguaqing.