A conventional organic light emitting diode, which has a structure of ITO/NPD/Alq3/LiF/Al, was fabricated and the magnetic field effects(MFEs) on its electroluminescence(EL) were measured. We found that the MFEs of the organic EL is strongly dependent on the applied bias on the device while the photoluminescence from the Alq3 film within the device is independent of the magnetic field. At low bias, the MFEs first rapidly increase with the magnetic field and then saturate at about 40kA/m, however at high bias the MFEs will decrease after this saturation, and the larger the bias, the stronger the decrease. The mutual annihilation of triplet-triplet pairs producing excited singlet excitons and the interaction between triplet excitons and excess carriers of Alq3 anion or cation are the physical mechanisms of the phenomena observed in this study.