Cu isotopes are a new geochemical tracer. A prerequisite for using this tracer to constrain some important geochemical processes is an adequate understanding of the processes and mechanism of Cu isotope fractionation. This paper reports the experimental results of Cu isotope fractionation during CuSO4·5H2O cry stallizing at room temperature, and reviews the current status of our understand ing of mass fractionation processes at low-temperature environments, including precipitation, reduction, adsorption, and biological process.