Laminar flow in a horizontal heated duct is characterized by a temperature gradient between the top and the bottom of a cross section due to mixed convection phenomenon. Wall temperature field is measured by infrared thermography. The aim of this paper is to present experimental results and analytical study allowing us to put in a prominent position some parameters which take into account the mixed convection phenomenon and, thereby, to describe the thermal behaviour of the duct. The numerical approach, using the Navier-Stokes equations coupled to the energy equation, completes this study showing, therefore, the secondary flow and the fluid temperature field in a cross section.