In this paper, an experimental investigation and comparative study were performed on the pressure drop and void fraction of air-water two-phase Bow in vertical helicoidal pipes. Eight coils have been tested. The results show that the pressure drop of the two-phase how in vertical helicoidal pipes depends on both the Lockhart-Martinelli parameter and the flow rates for low flow rates in small ratio of coil diameter to pipe diameter. The geometric parameters have no apparent effects on the void fraction, but they do affect the frictional pressure drop. Finally, the correlation for frictional pressure drop multiplier for small coils is provided based on the experimental data.