As a major rock- forming and highly incompatible element, potassium (K) is highly water- soluble and highly mobile. In recent years, the rapid development of multi- collector inductively coupled plasma- mass spectrometry (MC- ICP- MS) has greatly promoted the development of K isotopes due to the breakthrough of high precision K isotopic measurement. Till now, the K isotopic compositions of the major terrestrial reservoirs have been investigated, and behaviors of K isotopes during some geological and physicochemical processes have been studies. The results indicated that the K isotopic composition of seawater (averaging in 0. 11‰±0. 08‰) is obviously heavier than bulk silicate earth (BSE, estimated in -0. 44‰±0. 04‰). Large K isotopic fractionation occurs during continental weathering with light K isotopes retained in the weathered products and heavy K isotopes released into the water. And the fractionation during magmatic differentiation is limited. K isotopes have been used to trace the subducted crustal material in the mantle source of mantle- derive magmas. Since the K content of crustal materials is much larger than that of the mantle, K isotope is very sensitive to the metasomatism or assimilation processes in the mantle during the subduction processes. Thus, it is believed that K isotope would open new horizons to crust- mantle processes.