Pot trials were conducted to study the response of different fraction of B in rape leaves to different B fertilizer application levels in different growth stage and different position, and the possibility of using water extractable B in rape leaves as the diagnostic criterion was discussed.When B application level was higher than 5.1 kg/hm~2, both the biomass of rapes in seedling stage and shooting stage and the grain yield decreased, but the total B content and water extractable B content in leaves increased with the enhance of B application levels. Although there were good relationship between total B contents and B fertilizer application levels, the differences of total B contents among B fertilizer treatments were too narrow to be used as diagnoses;Good relationships was also appeared between water extractable B content in leaves and B fertilizer treatments, and the water extractable B content in both quickly-spreading leaves and well-spread leaves in seedling stage were sensitive to the B nutrient status, thus there was possibility to use water extractable B as the diagnostic criterion of B nutrition of rapes. But further research was needed for the setup of water extractable B level in leaves as the diagnosis criterion.