To clarify the effect of flow orientation on the gas-liquid interfacial structure and frictional pressure gradient in a narrow rectangular channel, simultaneous measurements of pressure gradients and spatio-temporal characteristics of gas-liquid interface are conducted for three different flow orientations ; vertical upward flow (V flow), horizontal flow between horizontal plates (H-H flow) and horizontal flow between vertical plates (H-V flow). The velocity field in the liquid flow with interfacial waves is also measured by the PIV and LIF methods. The interfacial structure at low gas and low liquid flow rates is greatly affected by the flow orientation, and the complicated interfacial structure is observed in V flow. To predict the frictional pressure gradient, the wave model which takes account of the wall shear stress for the wave propagation and base film flow is presented. From the comparison of experimental results with predictions by the present model, it is found that the difference in the frictional pressure gradient between the three flow orientations is closely related to the characteristics of interfacial waves as well as the average liquid velocity.