Two-phase flow Of Oil and water is commonly observed in wellbores, and its behavior under a wide range of fl;ow conditions and inclination angles constitutes a relevant unresolved issue for the petroleum industry. Among the most significant applications of oil-water flow in wellbores are production optimization, production string selection, production logging interpretation, down-hole metering, and artificial lift design and modeling. In this study, oil-waterflow in vertical and inclined pipes has been investigated theoretically and experimentally. The data are acquired in a transparent test section (0.0508 m i, 15.3 m long) using a mineral oil and water (rho(o)/rho(w) = 0.85, mu(o)/mu(w) = 20.0 & sigma(o-w) = 33.5 dyne/cm at 32.22 degrees C). The tests covered inclination angles of 90, 75, 60, and 45 deg from horizontal. The holdup and pressure drop behaviors are strongly affected by oil-water flow patterns and inclination angle. Oil-water flows have been grouped into two major categories based on the status of the continuous phase, including water-dominated and oil-dominated flow patterns. Water-dominated flow patterns generally showed significant slippage, but relatively low frictional pressure gradients. In contrast, oil-dominated flow patterns showed negligible slippage, but significantly large frictional pressure gradients. A new mechanistic model is proposed to predict the water holdup in vertical wellbores based on a drift-flux approach. The drift flux model was found to be adequate to calculate the holdup for high slippage flow patterns. New closure relationships for the two-phase friction factor for oil-dominated and water-dominated flow patterns are also proposed.