Effective length of swirl flow is important for its application in the industry, however the length of the swirl flow in a horizontal pipe is largely unexplored. In this paper, effects of swirl on a horizontal bubbly flow and its development in a 25 mm horizontal pipe with a length of 4 m were studied by experimental and theoretical method. In the experiments the superficial gas velocities ranged from 0.068 – 0.566 m/s and the liquid velocities ranged from 0.283 – 1.132 m/s. The results show that: instead of dispersed bubbles in the non-swirl bubbly flow, a gas column flow is transformed downstream of the swirler; then it breaks up into separated gas slugs, and finally transforms to a non-swirl bubble flow. The pressure drop in the swirl flow gradually decreases and approximates to that in the non-swirl flow along the streamwise direction. Transition mechanism for the gas column flow was proposed here, and the transition criteria of the gas column flow were developed based on Kelvin-Helmholtz instability, then the effective length of swirl on the gas column flow can be predicted.