The characteristics of heat transfer and flow on three-dimensional mixed convection in horizontal rectangular ducts with heated lower wall and cooled upper wall were examined numerically for five kinds of aspect ratios. The working fluid was water and the flow was assumed to be laminar and incompressible. The SIMPLE procedure and QUICK scheme were used to solve the governing equations through a control volume. The kinematic viscosity and the thermal conductivity of the water depend on temperature. The aspect ratios affected the generation and the number of the recirculated flows. The local and averaged Nusselt numbers corresponding to the flow were examined on the heated and the cooled walls, respectively.