Granite pegmatites are the most important source of lithium within hard- rock lithium resources. In recent years, numerous studies have investigated the diagenesis and mineralization processes in pegmatites using lithium isotopes. This review synthesizes the advancements in lithium isotope applications for pegmatites, focusing on their ability to: ① trace the source region of pegmatite melts; ② delineate melt separation, crystallization, and fluid exsolution events; and ③ provide insights into lithium metallogenesis. Researchers have demonstrated that crystallization differentiation and fluid exsolution processes in pegmatite melts cause significant lithium isotope fractionation. This fractionation manifests in several ways: ① crystallization enriches the melt in heavier lithium isotopes (δ7Li); ② fluid exsolution depletes the melt in heavier lithium isotopes; and ③ diffusion fractionation leads to the accumulation of 6Li at the distal end of the melt. A significant negative correlation exists between Li content and δ7Li in global pegmatite occurrences, including the Jiajika pegmatite, where increasing differentiation is negatively correlated with δ7Li. This isotopic lightening in differentiated pegmatite systems is likely attributable to several factors: extensive crystallization of 6Li- rich minerals like mica and spodumene, fluid exsolution, or dynamic fractionation, as previously documented. In addition, unbalanced crystal differentiation and hydrothermal alteration can induce significant isotope fractionation, thereby complicating the identification of lithium sources and hampering isotope tracing efforts. The lithium isotopic composition of pegmatites may not consistently reflect the origin of lithium sources. Nonetheless, it provides valuable insights into geological processes such as melt crystallization differentiation and fluid exsolution, contributing to a more comprehensive understanding of the melt- fluid evolution and lithium mineralization mechanisms within pegmatite systems.