Abstract: With air and water as the working fluids, an experimental study on the characteristics of the interfacial waves in gas - liquid two - phase flow in a horizontal rectangular channel was conducted by using three two - parallel conductance probes to measure the instantaneous liquid film thickness. The test section was entirely made of plexiglass plate and the length is 6690mm long. Its cross section was 150mm×25mm. In this experiment the maximum gas superficial velocity was 14.6m·s-1 and the water superficial velocity 0.110m·s-1. The wave pattern and flow pattern of gas - liquid two- phase flow were distinguished and defined, and compared with the results of previous studies. Furthermore, the characteristics of interfacial waves, such as wave speed, wavelength, wave frequency and wave amplitude were investigated in terms of the gas and liquid volumetric fluxes.