Efficiency and current measurements of ferromagnetic Co thin layer based organic light emitting diode, which has a structure of ITO/Co/NPB/Alq3/LiF/Al, are performed as a function of magnetic field at room temperature.The efficiency first increases by up to 18.8% as magnetic field increased to about 80 mT, and then appears to be almost saturated. The changes result from the spin-polaried injection of Co and the magnetic field effects(MFEs), both of which increase the fraction of siglets among the total excitons, and the former is primary. The changing rate of current through the device is found to increase by up to 6.9% at about 60 mT, and then decrease slowly with magnetic field increasing. These observations are explained in terms of the MFEs of polaron pairs dissociation and magnetic-field dependence of triplet-charge reaction in excited states.