The two-phase flow pattern and frictional pressure characteristics for R-134a inside a 6.5-mm (0.25-in.) smooth tube are reported in this study. The range of mass flux is between 100 and 500 kg/m 2 ·s, and the vapor quality is between 0.02 and 0.95. It is shown that the flow pattern map proposed by Weisman et al. (1979) gave the best agreement with the present experimental data. In addition, a correlation for the two-phase multipliers was proposed that can correlate the present test data with reasonably good accuracy.