Gas-liquid two-phase flow in a circular pipeline is commonly encountered in an inclined pipeline of an offshore plant. To visualize gas-liquid flow phenomena in an inclined pipeline, the w-shaped transparent pipeline was fabricated with internal diameter of 2″ and slope angle of 25°. The terrain-induced slug flow in a steady-state was visualized at fixed water flow rate of 1 m3/hr and 80% GVF (Gas Volume Fraction). The air and water flow is initially maintained in stratified or wavy flow at t = 0 s. When the velocity difference between the air and water is high enough, the Kelvin-Helmholtz wave motion starts to occur just after at t = 0 s. As the wave reaches the top, the upward water flow is faced with the downward water flow in the main visualized region. When the airway is clogged, the air slug is formed at t = 0.02 s. When a huge tidal wave is observed at t = 0.1 s due to different velocity between the upward water and the downward water flow, the air slug travels at a greater velocity than the water flow. As the tidal wave enlarges its growth, the chaotic motion with scattered bubbles is exhibited at the gas-liquid interface. A series of the air slugging is periodically observed after near 0.5 s. At the second v-shaped pipeline, the slugging phenomena become even more severe due to an irregular water inflow from the first v-shaped pipeline.