The present study deals with the secondary flow and heat transfer aspects of endwall boundary layer fences in 90 degrees turning ducts. Boundary layer fences have recently been re-introduced as additional components to achieve favorable aerothermal effects in turbine/compressor passages and internal cooling systems. The turbine passage or internal coolant channel is simulated by a 90 degrees turning duct (Re-D = 360,000) to study the aerothermal interaction of the boundary laver fence and the passage flow that is dominated by the passage vortices. Specifically, a single boundary layer fence of varying dimensions is attached to a heated endwall of the duct. The current study adds to the currently small number of investigations on the use of endwall boundary layer fences and their aerothermal interaction with the passage flow system.