|Effect of magnetic field on flow and heat transfer of an incompressible viscous fluid over a stretching horizontal cylinder in the presence of heat source or sink with suction/injection is discussed numerically. The governing boundary layer equations are reduced to a system of ordinary differential equations. Mathematica has been used to solve such system after obtaining the missed initial conditions. Comparison of obtained numerical results is made with previously published results in some special cases, and found to be in a good agreement. The boundary layer theory had been presented in 1904 by Prandtl. The problem of boundary layer flow and heat transfer over a moving or stretching surface is of interest in numerous industrial applications such as polymer extrusion processes where the object enters the fluid for cooling below a certain temperature, hot rolling, paper production, wire drawing, aerodynamic extrusion of plastic sheets, the boundary layer along a liquid film and condensation process of metallic plate in a cooling bath and glass. [Elbashbeshy EMA, Emam TG, El-Azab MS, Abdelgaber KM (2012) Effect of Magnetic Field on Flow and Heat Transfer over a Stretching Horizontal Cylinder in the Presence of a Heat Source/Sink with Suction/Injection].