An experimental study of the flow in a modeled hydraulic headbox of a papermaking machine was performed. The turbulent boundary layer generated on the partition plates in the dispersion part and the wake formed downstream of the plates were investigated by a flow visualization technique using the smoke-wire method and by hot-wire measurement. Control of the flow characteristics was examined for various combinations of two types of channel and four types of inserted plate that differ in convergence and trailing-edge shape, respectively. As a result, rectification by the use of a convergent channel was found to be effective in realizing a uniform time-averaged velocity distribution and in reducing and unifying the turbulence intensity. Furthermore, the trailing edges with tapered and wavy shapes were effective for obtaining a uniform time-averaged velocity distribution, and the tapered edge was highly effective for making the turbulence intensity a uniform distribution.