Pentacene-based metal-base organic transistors (MBOTs) are fabricated. The influence of the charge carrier injection efficiency at the emitter electrode/emitter interface on the device performance is investigated. It is found that the current modulation and the on/off ratio increase with the injection efficiency. By inserting poly(3,4-ethylenedioxythiophene) (PEDOT): PSS/m-MTDATA layers at the emitter electrode/emitter interface, the current modulation and the on/off ratio reach 6.7 mAcm−2 and 23, respectively. Meanwhile, the current gain is 95–96 in our experiment, which is almost independent on the injection efficiency.