Measured frictional pressure drops for single-phase water and two-phase air-water mixture flowing in nine small diameter U-type wavy tubes were presented. The inner diameters of the wavy tubes are 3.43 - 8.29 mm, whereas the curvature ratios span from 3.75~7.87. Test range of the Reynolds number for the single-phase water tests is about 200 < Re < 18000. The two-phase superficial velocities for air and water are in the range of UL,S = 0.02 - 0.7 m/s and UG,S = 0.2-60 m/s. The measured pressure loss in U-type wavy tube included the loss in U bends and the loss caused by the distorted flow in the downstream straight tube. This leads to the definition of the equivalent friction factor, fB and the two-phase pressure gradient in the U-bend. An empirical friction factor equation as a function of curvature ratio and spacer length, new Dean number and Re has been correlated from the present data with a bean deviation of 5.6%. The frictional characteristics of two-phase flows in U-type wavy pipes are also found related to the Lockhart-Martinelli relationship, the total mass flux and the curvature ratio (2R/D). However, the effect of the mass flux diminishes as the mass flux increases, G > 400 kg/m2·s and Martinelli parameter X < 0.3.