By using two-parallel conductance probes, the instantaneous film thickness of gas-liquid two-phase flow within a horizontal plexiglass pipe of 50mm inner diameter was experimentally measured. The pipe was 6680mm long. Several wave patterns were distinguished through statistical analysis of signals of film thicknesses. Wave pattern maps were obtained and compared with results of former studies. The characteristics of the interfacial waves, such as time-averaged film thickness, wave height, wave propagation speed, wavelength and wave frequency, were systematically investigated. The effect of the exit structure of the test section on interfacial waves was experimentally examined.