In this thesis an attempt is made to study the effects of magnetic field on some flows in channels and past infinite plates. In chapter 1 the effect of small uniform magnetic field on separation of a binary mixture for the case of fully developed natural convection of a fluid between two heated inclined plates is investigated.In chapter 2 free and forced convection flow of viscous incompressible fluid in a horizontal wavy channel has been studied. In chapter 3 Hall current effects on unsteady flow of viscous, incompressible fluid between two horizontal parallel non conducting plates, are studied. In chapter 4 the free and forced convection flow of conducting fluid between two parallel flat plates in an inclined channel is studied. In chapter 5 the effects of magnetic field, Heat generation and homogeneous chemical reaction on unsteady free convection and mass transfer flow of viscous conducting fluid past an infinite vertical porous plate is studied.