Spin injection in an organic semiconductor system under an external electric-field was studied theoretically by considering spin-dependent conductivity. The effect of the spin-dependent electrical-conductivity on the spin-injection efficiency is self-consistently derived. It is found that the spin-injection efficiency depends evidently on the spin-dependent electrical-conductivity. When the spin-injection efficiency increases 20%, the spin-dependent electrical-conductivity increases nearly four orders of magnitude at T = 10 K. With the increase of external electric-field, the spin-dependent electrical-conductivity increases sharply and makes the spin-injection efficiency increase, too. When the external electric-field varies from 1 to 10 mv/mu m, the spin-injection efficiency increases almost 10% at room temperature. It seems to be significant to consider the spin-dependent electrical-conductivity under the external electric-field in spin-injected organic system. (C) 2010 Elsevier B.V. All rights reserved.