Abstract: Aragonite at 0 to 70℃ was synthesized by slow decomposition a nd “two-step” overgrowth approaches. The phase compositions and morphology we re detected by XRD and SEM techniques. The effect of aragonite precipitation rat e on oxygen isotope fractionation between aragonite and water was studied by com bining XRD and SEM techniques with oxygen isotope analysis. For the calcium carb onates synthesized in the temperature range of 0 to 50℃, the XRD and SEM analyses show that with increasing temperature the rate of aragonite precipitati on increases and oxygen isotope fractionations between aragonite and water are p rogressively out of equilibrium. As a result, the experimentally measured oxygen isotope fractionations for the aragonite-water system at 50℃ is at disequilib rium, and the lower fractionation values obtained at 0℃ and 25℃ are proxy for equilibrium fractionations. Taking the lower values at 0℃ and 25℃ together wit h the data obtained from the “two-step” overgrowth technique at 50℃ and 70℃ , it yields the following fractionation equation for the aragonite-water system : 103lnα=20.41×103／T－41.42 This equation not only agrees well with the theoretic calculation by the increme nt method, but is close to the previous experimental results for the aragon ite-water and aragonite/calcite mixture-water systems as well as the empirical estimates on the biogenic aragonite-water system. It provides the first experi mental calibration of oxygen isotope fractionation between inorganically precipi tated aragonite and water at the low temperatures and is thus responsible for th ermodynamic equilibrium fractionation.