Abstract: By computing the drop of the quasi-Fermi level of majority carriers, the author obtained a rigorous boundary condition and thus the I-V characteristics for a forward-bias pn junction under high-level injection. The computed numerical results show that when W_p/L_H<3 (W_P is the width of the high-resistivity region, and L_H the effective diffusion length), the I-V characteristic follows the exp (qV_F/2kT) law, whereas W_P/L_H ≳ 3, the characteristic departs from this law, and even can not be expressed in the form of exp (qV_F/λkT).