Abstract: Rape (Brassica napus) No.2 Qinyou. the sensitive species to boron deficiency is chosen as test material. Shoot and root length. plant dry weight, leaves area and plasma memmbrane permeability were measured. Effect of optimum B supply on the plasma membrane permeability of isolated leaves with deficient supply of B under osmotic sttess were also measured. Boron function is studied under normal condition and osmotic stress. Result indicated that 0. 5mg/L is the optimum concentration to rape growth, no matter which concentrations are lower and higher than 0. 5mg/L. plasmma membrane function are hurted under osmotic stress. Optimum B concentration could reduce the hurt of osmotlc stress. improve its resistant to drought.