Local heat transfer coefficients and flow parameters were measured for air-water flow in a pipe in the horizontal and slightly upward inclined (2 degrees, 5 degrees, and 7 degrees) positions. The test section was a 27.9 mm stainless steel pipe with a length to diameter ratio of 100. For this systemic experimental study, a total of 758 data points were taken for horizontal and slightly upward inclined (2 degrees, 5 degrees, and 7 degrees) positions by carefully coordinating the liquid and gas superficial Reynolds number combinations. These superficial Reynolds numbers were duplicated for each inclination angle. The heat transfer data points were collected under a uniform wall heat flux boundary condition ranging from about 1,800-10,900W/m(2). The superficial Reynolds numbers ranged from about 740 to 26,000 for water and about 560 to 48,000 for air. A comparison of heat transfer data and flow visualization revealed that the heat transfer results were significantly dependent on the superficial liquid and gas Reynolds numbers, inclination angle, and flow pattern. The experimental data indicated that even in a slightly upward inclined pipe, there is a significant effect on the two-phase heat transfer of air-water flow. Flow pattern maps and flow visualization results for different inclination angles are also presented and discussed.