By considering the excitation of the spin wave at the interface between magnetic electrodes and insulating barrier and taking the mean-field approximation to the s–d exchange interaction, a T 3/2 law is derived for the temperature ( T) dependence of the exchange field in ferromagnets. Then we have investigated the T dependence of spin-polarized tunneling in a magnetic tunnel junction with an asymmetrical barrier. It is found that the extended Slonczewski model can provide a good approximation for the T-dependence of the junction magnetoresistance (JMR), a qualitative explanation for parallel tunneling conductance, and an exact description for the conductance difference between the parallel and antiparallel alignments of two electrodes ( ΔG). However, for the Julliere model, owing to the smaller contribution from interfacial states it involves, a decreasing T-dependence of the parallel conductance is derived, which is in contradiction with experimental results.