Laminar mixed convection heat transfer in a top and bottom heated rectangular channel with protruded discrete heat sources has been investigated experimentally for air. The lower and upper surfaces of the channel were equipped with 8 x 4 protruded heat sources subjected to uniform heat flux. Side walls, the lower and upper walls were insulated and adiabatic. An experimental study was made for Height/Width (H/W) ratios of (1/2), (1/4), (3/20) at various Reynolds (Re) and modified Grashof (Gr*) numbers. From experimental measurements, row-average surface temperature and Nusselt (Nu) number distributions of the discrete heat sources were obtained and effects of Reynolds and modified Grashof numbers on these distributions were investigated. The results show that the buoyancy affected secondary flow is more effective at the greater values of H/W ratios. (C) 2011 Elsevier Ltd. All rights reserved.