Abstract: A pot culture was conducted to study the effect of four B levels from B deficiency to excessive B on seed yield and quality of double low rapeseed cv. Huashuang 4 (HS-4), using double high rapeseed cv. Zhongyou 821 (ZY-821) as a control. The four B treatments were designed with B 0.3, 1.0, 2.5 and 5.0 mg/kg, respectively, with five replicates each treatment. The results showed that rate of B fertilizer from B 0.3-2.5 mg/kg could significantly increase the number of seeds per silique, siliques per plant and seed yield of the two cultivars. All of the three parameters were reduced when adding reached B 5.0 mg/kg, which indicated that excessive B would hamper the formation of seed yield. The extent of yield decrease is much more in HS-4 than ZY-821 at both B deficiency and toxicity, which showed that HS-4 is more sensitive to B deficiency and toxicity than ZY-821. Seed oil content and oleic acid content of the two cultivars presented an increased tendency, contrarily, protein content in seed showed a decreased tendency. These results suggested that: 1) rational application of B fertilizer would be more beneficial for increasing rapeseed seed yield than improving oil quality, and should be crucial for the production of double low rapeseed. 2) Feasible soil available B concentration could be heighten to B 1.0 mg/kg in rapeseed-planting region in China, however, application of B fertilizer must be controlled under B 2.5 mg/kg of soil available B concentration.