Establishing the relationship between temperature and δ 11 B value change is the key to understanding the boron isotope geochemical behavior of sedimentary borate during diagenesis- metamorphism. Natural tincalconite and ulexite used to study this relationship help to correctly understand the geological significance of δ 11 B value. When dehydration reaction of crystallization water occurs under 200℃, the δ 11 B values of the tincalconite and ulexite decrease from 6.48±0.14‰ to 5.41±0.27‰ and from -13.27±0.11‰ to -15.93±0.23‰, respectively. The 1‰～3‰ δ 11 B value change is consistent with the phenomenon observed in nature that the δ 11 B value of secondary borate formed by dehydration is 2‰～4‰ lower than that of primary borate. When the dehydration of hydroxyl group occurs between 200～300℃, it causes 1‰～2‰ δ 11 B value reduction, and the δ 11 B values of tincalconite and ulexite decrease from 5.41 ±0.27‰ to 3.73±0.26‰ and from -15.93±0.23‰ to -17.11±0.11‰, respectively. At 300~670℃, the borate undergoes amorphization and recrystallization, the δ 11 B values of the tincalconite and ulexite increase from 3.73±0.26‰ to 4.59±0.08‰ and from -17.11±0.11‰ to -16.08± 0.20‰, respectively. The 1‰ δ 11 B value increase is caused by the change of boron- oxygen framework. The results can indicate the formation process of the sedimentary borate and the variation trend of δ 11 B value during the burial process. The fluid with higher δ 11 B values is formed during the dehydration of borate, and may significantly affect the boron isotope composition of endogenous boron ore. Therefore, it is necessary to consider the boron isotope changes during fluid evolution when discussing the geological significance of boron isotope.