Abstract: A calcite crystal，sized in 4×6×10cm，was found in the calcite veins of the St. Andreasberg hydrothermal Pb-Zn deposits in the Harz Mountains，Germany. Detailed investigations of carbon and oxygen isotopic compositions were carried out at different locations within the calcite crystal in the following three drilling profiles：A.( 10l0)--(10 10): B (0001)--(000 1) and C.(0 110)--(1 101). For all of the three profiles，carbon and oxygen isotopes vary synchronously and both show evidently isotopic zoning，in which two zonations [( 1010)--(10 10) and (0001)--(000 1)profiles]are symmetrical and the other one[(0 l10)--( 1101)profile] is asymmetrical. According to theoretical modeling of the carbon and oxygen isotopic zonations of the calcite crysta1, three kinds of fluid are identified to be responsible for the formation of the isotopic zonations: A，subsurface fluid with temperature about 60℃，δ13C= -18.5‰ and δ180=0‰; B，deep-scaled crustal fluid with temperature about 140℃，δ13C= -7.0‰ and δ18O= +l0.0‰; C, meteoric-hydrothermal fluid with temperature, 1ess than 20℃，δ18O≤ 0‰ and δ13C≥ -14.0‰. The evolution history of the fluids effecting the calcite crystal can be divided into four stages: ① Calcite grew in fluid A with a constant temperature of 60℃; ② Fluid B mixed with fluid A homogeneously in a clewed system. At this stage，the temperature of the mixed fluid increased from 60℃ to 90℃ and δ13C and δ18O values increased from 18.5‰ and 0‰ to -16.3‰ and 3.8‰，respectively; ③ At the time when fluid B fluxed into the system slowly，fluid C was added instantaneously，which made the carbon and oxygen isotopic compositons and temperature of local fluid different from those of the predominant fluid. The difference was erased gradually after a period of time at the end. At this stage，the temperature of the mixed fluid decreased from 90℃ to 20℃ firstly and then increased to 80℃，and C value increased from -16.5‰ to -14.0‰ and δ180 value decreased from 3.8‰ to 0‰ then increased to 4.0‰ again; ④ Fluid B continued to mix with fluid A slowly，but the mixing system then became open so that the temperature of the system increased slowly to about 95℃ and then was kept at this temperature. and and δ13C and δ18O values of the mixed fluid increased to -10.7‰ and 7.0‰ at the end，respectively. The recognization of mixing of hydrothermn1 fluids in different fashions shed 1ight on the genetic mechanism of the St. Andreasberg ore deposits.