Goal of this experiment is knowing about influence of orifice's hole diameter variation to characteristics of down stream flow in circular ducting. The presence of orifice in channel system will cause any vortex at down stream region of flow, which was predicted could influence characteristics of flow in its moving to down stream direction. Vorticity which is contained by vortex will accumulate with flow vorticity, so that can influences produced boundary layer characteristics, involving boundary layer thickness and velocity profile. Influence of orifice's hole diameter variation to characteristics of down stream flow can be known by measuring static pressure and dynamic pressure along cross section to find out boundary layer thickness and velocity profile. The experiment results show that boundary layer develops faster at the smallest orifice's hole diameter, because vortex that happened at down stream region was the biggest.