The interfacial wave phenomenon in oil-water flow systems is an important area of research, due to its importance in real world applications, especially in oil-water transportation in petroleum industry. The influence of interfacial wavy flow and transition of flow regimes on pressure drop, hold-up and heat transfer has motivated the research on this topic to enhance the understanding, to ensure the process safety and to improve the process economy. This paper investigates the interfacial oil-water wavy flow in upwardly inclined pipes. The test fluids are mineral oil (viscosity-1.6 mPa s, density-788 kg/m(3)) and water. The scope of the study covers the upward pipe inclination angles of +3 degrees, +5 degrees, +6 degrees, mixture velocities of 0.2-0.5 m/s, and input water cut (input water volume ratio) 0.1-0.9. Two different flow patterns were observed in wavy flow in upwardly inclined pipes, namely stratified wavy (SW) and stratified wavy and mixed interface (SW&MI). The flow images were recorded using a high-speed video camera through a transparent test section. The image analysis was performed using several Matlab programs to extract wave properties such as wavelength and wave amplitude, as well as the wave speed. It is observed that the interfacial instabilities increase with the increasing mixture velocity and with increasing inclinations. Increased instabilities cause interfacial waves to generate and release droplets, while the turbulent intensity in the oil phase also influence droplet formation. An approximately linear relation between wave velocity and mixture velocity was obtained for the wavy flow and a correlation is presented accordingly. Wave energy manifests itself in the combined potential and kinetic energy. The potential energy via the wave amplitude and kinetic energy via the wave speed and wavelength. The overall energy for nonlinear breaking waves is a major source for generation of interfacial droplets. When the flow velocities are increased at a constant input water cut and at a given pipe inclination, the flow regime transition from SW to SW&MI occurs. Meanwhile, the prevailing wavelength decreases and the wave amplitude increases towards the point of transition from SW to SW&MI. The wavelength and the amplitude reach a critical value and remain constant until droplets start to form and release. Once the onset of drop formation occurs at the SW&MI flow regime, the wavelength starts to increase and the wave amplitude decreases with respect to their magnitudes at the point of transition. For a given velocity range, the mean amplitude increases with increasing inclination and decreases with increasing water cut. There is an inverse relation between wavelength and wave amplitude, which means higher amplitude always results in lower wavelength and vice versa. The wave velocity was calculated independently by two different analysis techniques applied to high-speed video images. One was carried out in space domain and one in time domain from high-speed image sequences. All data points were within the 7% error margin with respect to 1:1 reference correlation line, assuring the accuracy of analysis techniques and the validity of the correlation derived for relating the wave velocity to the mixture velocity. (C) 2018 Elsevier Ltd. All rights reserved.