Abstract: Resonant tunneling due to spin－dependent quantum well (QW) states in magnetic tunnel junctions have significant potential in research and applications. We discuss recent theoretical research on QW resonances in Fe(001)/MgO/Fe/MgO/Fe double barrier magnetic tunnel junctions. By calculating these QW states from first-principles and including the Coulomb blockade energy due to the finite size islands of the middle Fe film, we confirm that the oscillatory differential resistance observed in a recent experiment (Nozaki T et al. Phys. Rev. Lett., 2006,96: 027208 )originates from the QW resonances from the Δ1 band of the Fe majority-spin channel. The primary source of smearing at low temperatures is shown to be the variation of the Coulomb blockade energy.