Measurements of heat transfer and fluid flow of turbulent boundary-layer air flow in natural and mixed convection over an isothermal two-dimensional, vertical forward facing step are reported. The upstream and downstream walls and the step itself were heated to a uniform and constant temperature. Air velocity and temperature distributions and their turbulent,fluctuations are measured simultaneously using a two-component laser-Doppler velocimeter (LDV) and a cold wire anemometer, respectively. The present study treats buoyancy-dominated mixed convection over a vertical forward facing step and examines the effect of a small free stream velocity on turbulent natural convection. The experiment was carried out for a step height of 22 mm, for a range of free stream air velocities 0 m/sless than or equal tou(infinity)less than or equal to0.55 m/s (corresponding to a range of Reynolds numbers of 0less than or equal toRe(s)less than or equal to712), and a temperature difference, DeltaT, of 30degreesC between the heated walls and the free stream air (corresponding to a local Grashof number Gr(xi)=6.45x10(10)). It was found that the reattachment length increases while the heat transfer rate from the downstream heated wall decreases as the small free stream velocity increases.