Experimental investigations on the characteristics of air-water two-phase flow in the vertical helical rectangular channel are performed using the high speed flow visualization. The flow pattern map and the transition in the helical rectangular channel are presented. The flow pattern evolution in different positions of the helical rectangular channel is illustrated. The discussion on the coalescence of the bubble and slug is presented. The slug velocity, slug length distribution, liquid slug frequency, falling liquid film velocity and falling film thickness along the slug are investigated. The dimensionless liquid film thickness of the annular flow on the outer side of the channel is measured using the digital image processing algorithm. (C) 2015 Elsevier Ltd. All rights reserved.