A numerical code based on a finite volume technique was developed to simulate laminar flow over a three-dimensional horizontal forward-facing step. Pressure and velocity fields were linked by the SIMPLE algorithm. A rectangular channel encloses the forward-facing step such that the expansion ratio (ER) and the aspect ratio (AR) are equal to two and four, respectively. The channel total length in the stream-wise direction is equal to sixty times the step height and the step edge is located at a distance equal to twenty times the step height downstream of the channel inlet. At the channel inlet flow is considered to be fully developed and three-dimensional. Effect of variation of Reynolds number (Re=100, 200, 400, and 800) on locations of the reattachment line, the separation line, velocity profiles at different planes, and spanwise averaged Nusselt numbers are discussed.