To gain an insight into the effect of electric field on the electro-luminescence of organic light emission diode, the ultrafast pump-probe spectroscopy is used to investigate the transient phenomena of the excitons induced by the electric field. Under the excitation density of 230 μJ/cm2, the decay of the singlet excitons shows a fast and a slow component. The amplitude factor and the relaxation time of fast component are field-dependent. Compared with the zero biascase, the relaxation time of the fast component becomes faster at a bias of 6.4×105 V/cm, its amplitude factor is increased from 22% to 72%, about 50% initial excitons are dissociated by the electric field. The slow component is field-independent, its relaxation time-constant is about 890 ps. The longitudinal acoustic phonons with sound velocity of 17 /ps generated by excitation pulse are observed.