Mercury (Hg) isotopic compositions in hair and dietary sources from Wanshan (WS) Hg mining area, Guiyang (GY) urban area, and Changshun (CS) rural area were determined to identify the major Hg exposure sources of local residents. Rice and vegetables displayed low delta Hg-202 and small negative to zero Delta Hg-199, and are isotopically distinguishable from fish which showed relatively higher delta Hg-202 and positive Delta Hg-199. Distinct isotopic signatures were also observed for human hair from the three areas. Shifts of 2 to 3 parts per thousand in delta Hg-202 between hair and dietary sources confirmed mass dependent fractionation of Hg isotopes occurs during metabolic processes. Near zero Delta Hg-199 of hair from WS and CS suggested rice is the major exposure source. Positive Delta Hg-199 of hair from GY was likely caused by consumption of fish. A binary mixing model based on Delta Hg-199 showed that rice and fish consumption accounted for 59% and 41% of dietary Hg source for GY residents, respectively, whereas rice is the major source for WS and CS residents. The model output was validated by calculation of probable daily intake of Hg. Our study suggests that Hg isotopes can be a useful tracer for quantifying exposure sources and understanding metabolic processes of Hg in humans.