Interactions between neighboring wind turbines in complex terrain are complex and many problems are still unsolved. This paper, through wind tunnel tests, investigates the interaction between two horizontal axis wind turbines (HAWTs) under two different flow patterns. The purpose of the study is to begin to obtain fundamental procedures for arranging turbines in a wind farm in a complex terrain, with no-fixed intervals between wind turbines and different wind conditions. The main factors investigated are: (1) experimental analysis of the wake behind HAWTs, (2) performance measurements of the HAWTs, and (3) detailed investigation of interaction between two HAWTs. The results include the effects of inflow velocity, turbulence intensity and turbine array on wind turbine output power as affected by the wake.