Adiabatic, single-phase liquid and two-phase flow pressure drop were measured for R-12 flowing in both rectangular plain and micro-fin tubes with hydraulic diameters 2.64 and 1.56 mm, respectively. The single-phase liquid friction factors for the plain and micro-fin tubes are uniformly 14% and 36% higher, respectively, than that predicted by the Blasius equation. For two-phase flow, the pressure gradient increases with increasing mass velocity and vapor quality. The pressure gradient of the micro-fin tube is higher than that of the plain tube at same mass velocity and vapor quality. Predictive methods for the single-phase liquid and two-phase friction factor were also developed. These data are not well correlated by the Chisholm correlation which uses the Lockhart-Martinelli two-phase multiplier. However, the equivalent mass velocity concept proposed by Akers et al. provided a very good correlation of the present data. Both the plain and micro-fin tube data are correlated within ±20% by a single curve. This work shows that the pressure drop is dominated by vapor shear in both the plain and micro-fin tubes. Vapor shear effects in micro-fin tube do not cause significant disturbances in the two-phase flow. This observation provides additional evidence to support the conclusion in other work by Yang and Webb that the distinctly steep condensation heat transfer curves at low mass velocity and high vapor quality are caused by surface tension drainage force.