Megneto Hydro Dynamic (MHD) is very challenging problem to be analyzed the stability of Poiseuille flow. Therefore, in this work we investigate the instability of an electrically conductive fluid between two parallel plates under the influence of a transverse magnetic field. We apply the Chebyshev collocation method to solve generalized Orr-Summerfield equations to determine wave-number, growth rates, and spatial modes of eigen-modes. To get the neutral curves of MHD instability, the QZ method is used. It is observed that the magnetic field has a stabilizing effect on the flow and the stability increases as we increase the Hartmann number and for various wave numbers, magnetic field put down the growth of perturbation. It is concluded that there is a little effect of perturbations in span-wise direction for different Hartmann numbers that increase the critical values of Reynoldes numbers.