We have conducted molecular dynamics simulations of steady state water flow through (6,6), (7,7), (8,8), (12,12), (16,16), and (20,20) carbon nanotubes and analyzed the tube diameter dependence of microscopic flow structure at ambient temperature. Results show that the peaks of the atomic density profiles become more acute when a pressure gradient generates water flow through the nanotubes. In most cases, such profiles tend to shift closer to the tube wall compared to those in equilibrium conditions. We have also found that, using the velocities investigated here, the streaming velocity does not reduce the number of hydrogen bonds per water molecule regardless of the tube diameter.