Abstract: In the presence of a parallel magnetic field, the suppression of turbulent fluctuations in the plane channel filled with incompressible conducting fluids was investigated using direct numerical simulation method. The magnetic orientations concerned were the spanwise and streamwise directions, and the latter one was discussed mainly. The imposed streamwise magnetic field stimulates streamwise velocity fluctuations, while inhibits fluctuations in the other two directions. Relaminarization can be realized when the imposed streamwise magnetic field is sufficiently strong, but the transition from laminar to turbulence can not be observed by imposing three dimensional random background or impulse noises under the streamwise magnetic field.