With viscous dissipation and Joule heating taking into account a numerical solution of magnetohydrodynamic free convection flow, in the Stokes's problem, is obtained for different values of Prandtl numberP. The fluid is viscous, incompressible, and electrically conducting and the magnetic lines of force are assumed to be fixed relative to the plate which is started moving impulsively in its own plane (ISP) or it is uniformly accelerated (UAP). The solution is obtained with an implicit second-order method, forP=0.71 (air) andP=7 (water) and the obtained results are shown on figures and tables.