It is known that Fe is essential for life. Moreover, that the biosphere interacts with the lithosphere, hydrosphere and atmosphere, and biological action plays critical roles in the evolution of the Earth. Therefore, it is of significance to understand Fe isotope behavior during biological processes. In this paper, the authors summarize the Fe isotope compositions of various types of organism, and Fe isotope fractionation during biological activities. It is shown that organism takes up light isotopes preferentially, and the isotope fractionation occurs stepwise along the biological pathways. During the biologically induced processes (including Dissimilatory Iron Reduction and bacterial Fe oxidation), Fe actually does not enter the cell. The values of Fe isotope fractionation of these processes are consistent with those for non-biological reduction-oxidation. During biologically controlled processes (including microorganism, plants, animals and human beings), Fe does enter the cell. The Fe isotope fractionation during these processes is mainly controlled by the redox state of Fe. Fe isotopes have great potential applications in biology and medical science, and are likely to be a new tracer in these research fields.