Abstract: The stable boron isotopes are developed in recent years as an extremely powerful geochemical tool. Boron has two stable isotopes (10 B and 11B) and there occurs a large fractionation of boron isotopes in nature with δ11B values from －37‰ to +58‰. Those with very neg ative δ11B samples are from non-marine evaporites and some tourmalin es， whereas those with very positive δ11B samples are salt-lake bri nes and evaporitic seawater. Meteorites show a large variation in δ11B values (－50‰ to +44‰)， but the δ11B variations in lunar ro cks are relatively limited (－6‰ to +4‰). The δ11B values of v olcanic rocks vary from －15‰ to +21‰ depending on the rock type and t he degree of crust contamination， and the primitive mantle has been estimated h aving a δ11B value of －10‰±2‰. The boron isotope composition s of metamorphic rocks vary with metamorphic grade and their protoliths. Modern oceanic sediments have δ11B vaues of －6.6‰ to +4.8‰， w hereas ancient sedimentary rocks are slightly depleted in 11B with an limited δ11B database of －17.0‰ to －5.6‰. Tourmalines from granites and pegmatites and from massive sulfide deposits have δ11 B vaules of －37‰ to +0.8‰， －25‰ to +10‰， and －23‰ to +18‰， respectively. Modern oceanic water has a rather constant δ 11B value of +39.5‰. Submarine hydrothermal fluids show a δ 11B range from －2.6‰ to +36.8‰， depending on the tectonic se tting of the vent fluids. Subaerial geothermal fluids and groundwater have gene rally lower δ11B values than seawater and vary considerably with diff erent settings and country rocks. 　　Boron has three major geochemical characteristics: (1) very soluble. The majori ty of boron occurs in rocks and waters in crust and hydrosphere. During water/r ock interaction， boron is highly mobile. (2) very large fractionation between the two isotopes of 10B and 11B due to their relatively larg e mass differences. (3) lack of complexity associated with oxidation/reduction reaction. The dominant boron forms in natural solutions are B(OH)3 and B(OH) 4－， their relative concentrations and δ11B values are pH-depen dent. 　　These features set boron significantly apart from other stable and radiogenic is otopes and make boron a unique and sensitive geochemical tracer. Recently boron isotopes have found its major applications in the following fields: (1) cosmoch emistry and star formation processes; (2) crust-mantle evolution and subduction -related processes; (3) sedimentary environments and the source of boron; (4) h ydrothermal ore-forming processes and ore-genesis; (5) paleo-ocean pH estimat ion and paleoclimatology; (6) groundwater research and environmental geochemistry.