By means of simple one-band tight-binding Hamiltonian and using the diagrammatic tech-niques for nonequilibrium processes proposed by Keldysh, we investigate the resonant tunnel-ing transport properties through GaAs/Al_xGa_1-xAs double-barrier heterostructures under the ac-tion of magnetic fields applied parallel and perpendicular to the current direction. We have found that the number of Landau levels that contribute to the resonant tunneling diminishes with the magnetic field applied parallel to the current direction. Also, we have found that the intensity of the resonant peaks increase when the magnetic field is applied parallel to the cur-rent direction. The magnetic field applied perpendicular to the current direction leads to a shift to higher voltage and the diminishing of the intensity of the resonant tunneling peak in the characteristic curves of current versus voltage. Also, we have found that the diminishing of the hopping energy between nearest neighbors originates the same effects than the in-plane mag-netic field on the current-voltage characteristics. Our results compare quite well with experi-mental reports.