We report the magnetic-field effect (MFE) in Alq3 based organic light-emitting device (OLED) with the structure of ITO/NPB/LiF/Alq3/LiF/Al.When an external magnetic field of 50 mT and insertion of LiF thickness of 0.8 nm is applied, the current efficiency and the brightness of the device increase to 12.4% and 17%, respectively.The origin of the improvements could be attributed to the Zeeman effect,which reduces the hyperfine interactions of electrons and holes, inhibiting energy transfer from the singlet polaron pairs to triplet polaron pairs and increasing the population of singlets.Similar MFE was observed in phosphorescent organic light-emitting diodes(PhOLEDs).The current efficiency of ITO/NPB/LiF(0.8 nm)/CBP:6 wt% Ir(ppy)3/BCP/Alq3/LiF/Al increases to 12.1% at 50 mT.Magnetic-field-induced increase of singlet excitons in the CBP matrix reflects increase of triplet excitons in the dispersed Ir(ppy)3 molecules in the CBP+Ir(ppy)3 blend.