Abstract: Experimental investigations on air-water two-phase flow in a vertical circular channel and three noncircular channels（an equilateral triangular channel and two square channels） were carried out.The hydraulic diameters of the channels are 14mm,14.43mm,15mm and 10mm,respectively.Superficial air ve-locity and water velocity are about: V_SG =0.04~80m/s,V_SL=0.001~6 m/s.The following typical flow patterns for the channels at different air and water superficial velocities were recognized by flow visualization: bubble flow,slug flow,churn flow,annular flow,dispersed bubble flow,and creep flow（or rivulet flow for the cir-cular channel）.The observation of creep flow in noncircular channels has proved that the influence of the channel geometry results in the steeper radial distribution of the phases and velocities,and eventually turbu-lent secondary flow,which affects two-phase flow patterns in noncircular channels at high air superficial velocities.The secondary flow in a triangular channel is stronger than that in a square channel,it proves that the influence of the channel geometry is of significance.The flow pattern maps and flow pattern transition boundaries were obtained.Comparison with the previous literatures shows that the influences of the channel geometry and hydraulic diameter on the flow pattern transition do exist.