The injection of polymer solution into turbulent pipe flow caused significant drag reduction. The injected polymer threads were visualized photographically. Combining the flow visualization results with the measurements of drag reduction caused by the centerline injection, the possible interaction between polymer threads and turbulent eddies was discussed. It was proposed that, for the centerline injection system, there existed the maximum drag reduction asymptote which was different from the maximum drag reduction asymptote for premixed polymer solutions. This was confirmed by evaluating the pipe diameter effect and the concentration effect in the centerline injection system. The annuler injection in the near wall region caused more significant drag reduction than those obtained for the centerline injection. The maximum drag reduction by annuler injection coincided with the maximum drag reduction asymptote for the premixed polymer solutions.