Abstract: A laboratory experiment, along with theoretical analysis, was conducted to study the vertical profiles of flow velocity on the center plane of an open channel with a bottom sill, and from the measurements a parabolic formula was established for the profiles at the crest of sills with different shapes. Results show a linear relationship of the parabola coefficient versus the slope of the upstream sill face. And, the peak of the velocity profiles moves closer to the channel bottom as this sill slope increases, with its lowest position at a bottom distance as small as 41% of the flow depth, i.e. far away from the water surface. The peak value of velocity is increased with the increasing flow rate, but the ratio of peak to mean velocity approaches to constant. Calculations of this formula agree with the measurements and its accuracy is higher for lower velocity, showing it effective in description of the actual velocity over the sills.