Two-phase frictional pressure characteristics for R-410A and air-water mixtures inside a 5 mm smooth tube are reported in this study. The range of mass flux is between 100 and 700 kg/m2·s, and the vapor quality is between 0.05 and 0.95. The test results indicated that neither C = 5 nor C = 21 of the Chishom (1973) correlation can describe the test results. The Friedel (1979) correlation also failed to predict the present test results. It is speculated that the Friedel correlation may underestimate the effect of surface tension and overestimate the influence of gravity for small diameter tubes. Hence, a slight modification to the Friedel correlation is proposed that can extend the capability of the Friedel correlation in the small diameter range.