LtdAn experimental study of three-phase flow in a vertical pipe with 50 mm diameter has been carried out. The experiments were conducted under the input superficial phase velocity: water from 0 to 0.885 m/s, oil from 0 to 0.90 m/s and gas from 0 to 0.85 m/s. In order to investigate the influence of gas injection on an oil-water two-phase flow, the average in situ phase fraction and pressure gradient were measured. The results showed that gas injection had little impact on the phase invasion and the phase inversion was still taking place at the same oil volume fraction. The average in situ gas fraction reached to its lowest value around the phase inversion point. In comparison to oil-water flow, the presence of gas decreased considerably the gravity pressure gradient and the total pressure gradient was reduced. However, when the input oil fraction was close to the phase inversion point, the contribution of frictional and gravity terms to the total pressure gradient became more equal, hence the total pressure gradient in three-phase flow could be higher than that in oil-water flow around the phase inversion region. In addition, several methods based on an oil-water flow as a single phase flow were evaluated to predict the pressure gradient of three-phase vertical bubbly flow. A good agreement was obtained between theory and data in this work and those in the literature.