Electrofluorescent organic light-emitting device with the structure of ITO/NPB/Alq3/LiF/Al and electrophosphorescent device with the structure of ITO/NPB/CBP: Ir（ppy）3/BCP/Alq3/LiF/Al were fabricated. Their electroluminescence properties were measured under an external magnetic field of 50 mT. Compared to the case of without magnetic field, the electroluminescent（EL） efficiencies of the above-mentioned two kinds of devices increase 44% and 7%, respectively. Based on the luminescence mechanism of organic light-emitting diodes （OLEDs）, the observed phenomena were explained. The origin of the improvement can be attributed to the Zeeman split of triplet state due to the external magnetic field influence, resulting in the conversion of triplets T+ into singlet S in the electrofluorescent device and an increase of the rate constants of triplet energy transfer from host to guest in electrophosphorescent device, respectively. At the same time, the reduced concentrations of triplet decreases the interaction of triplets with free carriers, resulting in an increase in the mobility of free carriers within the device and hence increasing the current.