In this work, we theoretically study the spin-dependent transport in a magnetic tunnel junction (MTJ). Using a simple model and ballistic transport, the magnetic polarization of the tunneling current on this system is studied by focusing on the tunneling of s and d electrons. We investigate the tunneling of these electrons through potential barriers, which represents the insulating layer between the ferromagnetic electrodes. We also examine how the conductance depends on voltage applied between the electrodes and on the effective mass of the electrons. The conductance is controlled by the transmission coefficient of the tunnel effect, and qualitatively it is known that tunneling probability of the d electrons is lower than the s electrons. We also estimate the effect of the tunneling magnetoresistance (TMR) and it is strongly influenced by the effective mass of the electrons. The s electrons do not contribute significantly to the TMR. © 1965-2012 IEEE.491256355638Prinz, G.A., Magnetoelectronics (1998) Science, 282 (5394), pp. 1660-1663Wolf, S.A., Awschalom, D.D., Buhrman, R.A., Daughton, J.M., Von Molnar, S., Roukes, M.L., Chtchelkanova, A.Y., Treger, D.M., Spintronics: A spin-based electronics vision for the future (2001) Science, 294 (5546), pp. 1488-1495. , DOI 10.1126/science.1065389Tsymbal, E.Y., Mryasov, O.N., Leclair, P.R., Spin-dependent tunnelling in magnetic tunnel junctions (2003) J. Phys.: Condens. Matter, 15 (4), pp. R109Li, Y., Qian, F., Xiang, J., Lieber, C.M., Nanowire electronic and optoelectronic devices (2006) Materials Today, 9 (10), pp. 18-27. , DOI 10.1016/S1369-7021(06)71650-9, PII S1369702106716509Kokado, S., Harigaya, K., A theoretical investigation of ferromagnetic tunnel junctions with 4-valued conductances (2003) J. Phys.: Condens. Matter, 15 (50), p. 8797Julliere, M., Tunneling between ferromagnetic films (1975) Phys. Lett. A, 54 (3), pp. 225-226Moodera, J.S., Kinder, L.R., Wong, T.M., Meservey, R., Large magnetoresistance at room temperature in ferromagnetic thin film tunnel junctions (1995) Phys. Rev. Lett, 74 (16), pp. 3273-3276. , ArLandauer, R., Electrical resistance of disordered one-dimensional lattices (1970) Philos. Mag, 21 (172), pp. 1478-6435. , NovCallaway, J., Wang, C.S., Energy bands in ferromagnetic iron (1977) Phys. Rev. B, 16 (5), pp. 2095-2105. , SepBratkovsky, A.M., Tunneling of electrons in conventional and halfmetallic systems: Towards very large magnetoresistance (1997) Phys. Rev. B, 56 (5), p. 2344Ashcroft, N.W., Mermin, D.N., (1976) Solid State Physics, , 1st ed. Toronto, ON, Canada: Thomson Learning