This thesis presents an experimental study of laminar flow mixed convention heat transfer in a bottom heated and top cooled horizontal duct, using air as the medium of heat transfer. The research plays an important role in the design and operation of several industrial equipment involving heat transfer and fluid flow phenomena. The lower surface of the channel were equipped with heat sources subjected to uniform heat flux and the upper surface of the channel were incorporates a milled-channel to circulate constant temperature water provide to act as a constant temperature sink.