The flow over a backward facing step mounted in a two-dimensional channel was investigated numerically, and was especially examined for the zone of recirculating flow. A stream function-vorticity formulation for the flow equations was used, and was solved by the finite difference method using the pseudo-unsteady technique. The calculation was performed for the range of step Reynolds number (the Reynolds number based on the step height) of 25≦R_≦250,and for the range of step ratio of 0.1≦h_s≦1.0. The results of calculation are reported, and are compared with other numerical and experimental results. The present study also includes the experimental results utilizing the flow-visualization technique by the authors, and an example (R_=40) of those results is presented in this paper. Good agreement was obtained between the experimental and numerical results. In most of the available work, the step Reynolds number was used as a single parameter that defined the reattachment length (the length of recirculationg flow zone) in the laminar two-dimensional backward facing step flow. However, B. F. Armaly et al. (1983) suggested that the reattachment length in laminar two-dimensional backward facing step flow was not a function of a single variable but was more likely to be a function of three or more variables, including the step or expansion ratio, the step or inlet-section Reynolds number and the velocity profile at the inlet section. In this paper, this suggestion was examined by utilizing the numerical and experimental results.