The organic light emitting diodes with the structure of ITO/NPB(60 nm)/ Alq3 ∶Rubrene(1wt%,20 nm)/Alq3(3 nm)/Alq3:Rubrene(1wt%,20 nm)/ Alq3(20 nm)/LiF/Al, which have multiple quantum well structures (MQW), were fabricated and the magnetic field effects on the efficiency and current of the OLEDs were measured. The experiment showed that the current decreased monotoniocally, i.e. the resistance of the device increased under the magnetic field. At the same time, the magnetic field effect on efficiency was achieved. The changing ratio of efficiency increased 9.13% maximally when the magnetic field was below 20 mT. The changing ratio of efficiency decreased with the magnetic field increasing when the magnetic field was higher than 20 mT.