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Oilseed rape (<i>Brassica napus</i> L.) is one of the most important oilseed crops. It has relatively high boron (B) requirements for growth. In this study, a hydroponic experiment was performed to determine the critical B requirement and B distribution in <i>B. napus</i>. The plants were grown for four weeks at a range of B levels (from 0.25 to 1000 µM) supplied in a nutrient solution. The results showed significant differences in the root and shoot dry matter and B accumulation in these tissues among the supplied B levels. Severe visible symptoms of B deficiency were observed on the leaves at levels lower than 1 µM B and toxicity at 1000 µM B in the nutrient solution. The maximum shoot and root dry matter were recorded at 25 µM B in the nutrient solution. The plants supplied with the lowest and the highest B levels produced 35% and 37% less shoot dry matter than those supplied with 25 µM B, while the corresponding decreases in the root dry matter were 48% and 36%, respectively. The critical concentration of B, which is the lowest concentration at which plants produce 90% of the maximum shoot dry matter, was proven to be 1 µM B for oilseed rape. At this level of external B supply, the B concentration in the shoot was 26.9 mg kg⁻¹ DM. It was found that with the increase in B levels in the nutrient solution, the relative distribution of B between the roots and the shoots shifted in favor of the shoots.