Experiments were performed to investigate the friction pressure drop of upward air-water two-phase flow in vertical twisted-tape-inserted tubes. Total pressure drop and average void fraction for hydrostatic pressure drop were measured at the same time, from which the friction pressure drop was calculated. The friction pressure drops of twisted-type-inserted tubes were 1.2∼5 times as large as that of empty one. However, the two-phase friction multiplier of the former was smaller than that of the latter and decreased with decrease of twist ratio (more tightly twisted). Such trends were predicted well by the well-known Chisholm's equation with modified value of the constant C based on the present experimental data. A correlation for the constant C was obtained as a function of twist ratio, with which all the present experimental data of friction pressure drop in the twisted-tape-inserted tubes were predicted well within ±30 percent error range.