Recent attempt in geochemical and isotopic investigation at Fang geothermal area has been done by staff of Geothermal Working Group of Thailand under a support in isotopic determinations and sampling facilities from the International Atomic Energy Agency (IAEA) though a regional TC project (RAS 8075). The investigation is able to indicate and confirm the upflow part of the system southeast of manifestation area whereas most shallow wells at Fang appear to be sitting over the outflow. Reservoir temperatures of 170^oC and geochemical modeling of the thermal fluids are determined by using WATCH 2.1. Computer simulation indicates a low pH (less than 5.5) for deep thermal water. At the shallow reservoir, boiling at about 120^oC increases the pH to 8.0 or more due to the loss of carbon dioxide. Subsequently after boiling, the thermal water tends to be oversaturated with respect to calcite, fluorite and quartz. An Oxygen shift among the thermal water samples has been observed in this study and is referred to isotopic exchange with wall rocks. The different enrichment of d ¹⁸ O between shallow and deep wells might be due to different in residence time of the water in exchange reactions. Since it is apparent that shallow well water samples collected near upflow area have their isotopic composition more or less similar to local meteoric water whereas the deeper wells far away are well enriched in d ¹⁸ O. A geothermal conceptual model is proposed in order to explain all data observed. The main geothermal reservoir of Fang, which received water from local precipitation, is probably situated at depth less than 2 km. due south of the study area. Thermal water from this deep reservoir has probably flown northwards to the manifestation area via intersection of local faults. It is also suggested that the deposition of fluorite at a nearby mine might have been formed by interaction of the fluoride rich thermal fluid and limestone wall rock.