The tunneling conductance (TC) and magnetoresistance (TMR) are investigated for magnetic junctions consisting of two ferromagnetic electrodes separated by a ferromagnetic insulator (semiconductor). The investigations are based on the nearly-free-electron approximation. The results show that TC and TMR strongly depend on the magnetization configuration of the junction such that TC and TMR reach their maximums when the magnetic moments of the two electrodes are parallel to each other but antiparallel to that of the barrier, whereas the minimums appear when the magnetic moments of both electrodes and the barrier are parallel. The tunneling probability (TP) relates to the spin orientation of incident electrons and the magnetization configuration of the junction. The variation of TC and TMR with the relative orientation of magnetization of both electrodes and the barrier can be explained by two effects: the ferromagnet-ferromagnet tunneling and spin-filtering effect. Our results agree well with experiment.