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Highlights • The liquid film behavior at the transition to slug flow in horizontal pipes were studied experimentally. • It was obtained from the visualization studies, the pressure gradient and the liquid hold-up measurements. • The wave growth and the wave coalescence are the responsible mechanisms for slug formation. • The wave growth exists at low J L and J G , meanwhile the wave coalescence exists at a higher J G. Abstract The knowledge of the slug flow is essential the design safety of hydrocarbon transportation in the pipeline system of petroleum industries. However, the suitable models to predict the transition condition to slug flow are limited due to insufficient availability of experimental data. In the present study, the hydrodynamic characteristics of the near the transition to slug flow of air-water two-phase flow in horizontal pipes were investigated experimentally for inner pipe diameters of 16 mm, 26 mm, and 50 mm. The superficial velocities of water and air ranged from J L = 0.03 to 0.30 m/s and J G = 0.7 to 10.0 m/s. The hydrodynamic behavior was obtained from the combination of the visualization detected by using a high-speed video camera, the pressure gradient, and instantaneous volumetric liquid fraction measurements. The experimental results indicate that the wave growth and the wave coalescence are involved in the mechanism of the slug flow formation. Both gas and liquid superficial velocities have a significant effect also of those mechanisms. The average volumetric liquid fraction near the transition to slug flow is not affected by superficial gas velocities. Next, under a constant superficial gas velocity, an increase in the inner pipe diameter requires a larger superficial liquid velocity to change the flow patterns from stratified smooth to slug flow, and from pseudo-slug to slug flow. [ABSTRACT FROM AUTHOR]