Laminar mixed convection heat transfer in a top and bottom heated rectangular channel with protruded discrete heat sources has been investigated numerically for air. The lower and upper surfaces of the channel were equipped with 8 x 4 protruded heat sources subjected to uniform heat flux. Results show that surface temperatures increase with increasing Gr(*) numbers. For high values of Gr(*) numbers where natural convection is the dominant heat transfer regime (Gr/Re-2 > 1), temperatures of heaters can have much higher values. In this region while the row -averaged Nu numbers of top heaters decrease with the row number, the row -averaged Nu numbers of bottom heaters decrease until a certain row. After this row however, there is an increase in the row -averaged Nu number due to the effects of natural convection. As a result of this while the temperatures of top heaters increase with the row number, the temperatures of bottom heaters decrease after the certain row.