Magnetohydrodynamics is a new branch in continuum mechanics which deals with the fluid flow affected by the magnetic field, and investigates the interaction of magnetic fields and electrically conductive fluid. In the most important industries, the magnetic field is widely used.For example, in the manufacturing industry, for heating, pumping, mixing and flotation molten metals as well as the weakening of free movement of the molten streams, strengthening the flow in channels with small dimensions and even creating flow in micro-channels are used. In this thesis, the effect of the uniform magnetic on the flow field, heat transfer and mixing has been studied. Using finite volume method continuity, momentum, energy and concentration equations were solved. Four cases of applying the magnetic field are considered. In each case, velocity, temperature and concentration diagrams in output section of the channel is represented. It is observed that the velocity, temperature and concentration field have been affected by the magnetic field. It is also observed that considering the electrical conductivity as a function of the concentration in parts of the channel that the concentration is higher, by applying a magnetic field velocity decreases and this is due to the Lorentz force. As the Lorentz force is increased,the velocity of flow is reduced. Moreover, velocity, concentration and flow contours, velocity vector, Nusselt number, Poiseuille number and mixing parameter for different modes of applied magnetic field is drawn. For the case where the magnetic field is applied on a permanent basis and to the whole channel, Nusselt and Poiseuille number is strongly influenced by the magnetic whichy increasing the maximum Hartmann number, Nusselt number increases on the lower wall and is reduced on the upper wall. Furthermore, by increasing maximum Hartmann number, Poiseuille number on both channel walls increases while changes on the lower wall of the channel is more than the upper wall. When the magnetic field is applied permanently to the channel, mixing parameter increases with increasing Hartmann number, but mixing occurs only at the interface and is not significant. It is observed that in the case of applying the frequency magnetic field in, in comparison with applying the permanent magnetic field, mixing take place in a better way. Key words : Magnetohydrodynamics, Heat Transfer, Mixing