Lithium has two stable isotopes, 7Li and 6Li, which have the biggest relative mass difference among all isotope pairs except for hydrogen-deuterium. Its potential for mass-dependent fractionation is thus obvious. Unlike the better established light stable isotopic systems (e.g. C, O, S), Li is a trace cation and does not form an integral part of the hydrological, atmospheric or biological cycle. Hence measurements of Li isotopes are likely to provide information rather different from data obtained from more commonly used isotope systems. Lithium isotope is regarded as a potential trace element because Li has many favorable characteristics, such as moderate incompatibility during mantle melting, high mobility in hydrous fluids and a strong isotopic fractionation at low temperatures, which make this trace element a powerful tracer for recycled materials in the mantle. In recent years，studies of Li and its isotopes in the solid earth have been rapidly increasing in number, as there exists great interest in determining the usefulness of the Li isotope system in tracing crust-mantle recycling. Lithium isotopes can be strongly fractionated under different conditions, with δ7Li values ranging from +32‰ in sea water to very low in eclogites（-35‰）, which are interpreted as analogs of the dehydrated oceanic crust. Because Li is a fluid-mobile element, it has been held that heavy seawater Li incorporated into altered oceanic crust would move from the slab into the mantle wedge together with other fluid-mobile elements during subduction zone metamorphism and ultimately manifested itself again in island arc lavas. However, studies of island arcs show that Li is decoupled from other fluid-mobile elements, and most island arc lavas (+2‰～+6‰) have δ7Li indis tinguishable from δ7Li of MORB. Several models have been proposed to explain the Li isotopic composition of lavas, such as fluid filtering and fluid-mantle wedge mixing. Thus, the isotopic composition of recycled Li remains a matter of speculation in the subduction zone.