In this study, the effect of the flow routing plates on the laminar mixed convection heat transfer in a horizontal channel that has protruded heat sources at the bottom and top surfaces were investigated numerically and experimentally. The air was used as the cooling fluid, and protruded heat sources were equipped as 4x8 rows into the rectangle channel that has insulated walls. The experimental study was applied for two different Reynolds (Re) numbers. A numerical model complying with the experimental results was created, and numerical investigations were performed in different Reynolds and modified Grashof(Gr*) numbers for the 0, 30, 60 values of the plate angles (a). The analyses showed that using flow routing plate only increases the heat transfer from the first four heater rows on the bottom surface, and the first and the last heater rows on the top surface. The findings obtained during the experimental and numerical studies were presented in detail as graphics showing the row averaged Nusselt number (Nu(row ave)) the heater temperatures, velocity vectors, and temperature contours.