In areas where soils are low in boron (B), the ability to predict B deficiency by plant analysis depends on the development of calibrated standards and offers the potential of preventing or minimising losses in seed yield. The present study aimed at establishing relationships between seed yield in winter oilseed rape ( Brassica napus ) and B concentrations in defined leaves during growth before fiowering and from them to determine critical values for the prognosis of B deficiency. Oilseed rape cv. Zheyouyou 2 was grown in 10 field experiments in Zhejiang province, south-east China, with B rates from 0 to 3· 3 kg/ha. At 4 of the sites, oilseed rape was resown in the following year to assess seasonal effects on critical concentrations. Experiments were conducted on the 3 main soil groups on which oilseed rape is grown in south-east China. The youngest open leaf (YOL) was selected as the sampling part because its B concentrations were strongly responsive to increasing B supply and well correlated with seed yield response. In this study, the best model of the relationship between YOL B concentration and yield was the Mitscherlich equation, which explained 68-89% of the variation in the data. The calculated critical range in the YOL at seedling stage was 20-25 mg B/kg and appeared to be valid in 2 seasons and on 3 soil types. The critical B range for prognosis of B deficiency was found to predict consistently those crops in farmers" fields that had low seed yield and low soil B, but needs to be evaluated in other growing environments especially those for spring rape.