It is well known that sediment particles introduced in open-channel turbulent flows change mean velocity profile, since Vanoni suggested the reduction of the Karman constant in 1946. However, how the sediment particles take such a role and what parameters would be changed have been debated up to now. Some researchers, on the other hand, have insisted that the constant would not be changed regardless of introducing sediment particles. The present study is a careful re-evaluation of the previous studies on this issue. The study revealed some questionable approaches or methods in the decision of the previous researches and found the reason why this issue has been debated for a long time. The result indicated that the Karman number is reduced by adding sediment particles, but the amount of reduction is much smaller than the previous researches insisted. Finally, the present study proposes a mechanism of the Karman number reduction due to sediment particles.