Buoyancy induced transient transitional flow structures and the associated heat transfer in mixed convective aiding flow of air in a bottom heated inclined rectangular duct were numerically investigated. The three-dimensional unsteady Navier-Stokes and energy equations were discretized by higher order finite-difference approximations and directly solved by the projection method. Computations were carried out for the Reynolds number ranging from 100 to 500, aspect ratio of the duct from 2 to 4, duct inclination from 0 degrees to 60 degrees C and buoyancy-to-inertia ratio high enough to cause flow transition. Attention was particularly paid to delineate the effects of the duct inclination on the flow transition and the associated temporal and spatial how structures. It is of interest to note from the predictions that increasing the inclined angle measured from horizontal does not always stabilize the flow. In fact, the flow stability is highly dependent on all the governing parameters and is intimately related to the structural changes in the buoyancy induced longitudinal vortices.