Abstract: Pipe friction is a teaching content in the course of fluid mechanics, in which Darcy friction factors in transition zone obtained by using artificial and commercial pipes are different. The present work shows analyses on the difference at two aspects. One is according to the understanding of students on the problem, and another is from the definition of surface roughness. Both show the surface roughness effect on the phenomenon. It is still necessary to research the quantitative effect of surface roughness on the difference between artificial and commercial pipes in transition zone.