This study is an experimental investigation of two-phase water-air flow in hilly-terrain duct.The inclination angles for hill and valley configuration is ±7.5o. Review of the related literature showstheir results are limited to slug regime only. In the present study, flow regime map and pressure traces are investigated. This study reveals that the possible slug flow behavior categories exist along a valley and hill configuration are four and two, respectively. In an attempt to relate the qualitative flow behavior at a valley and hill to the flow pattern maps of upstream and downstream, this qualitative classification is superimposed on flow pattern maps where obtained independently for the upstream and downstream sections. The results show, flow has different behavior in hill in compare to valley at relatively low gas flow rates. However, at higher gas velocity, difference between hill and valley behavior decreases.It can be concluded, that the effect of hill and valley behavior are similar on flow regime at relatively high gas flow rates.