The present study investigates rotation-induced secondary flow and mixed convection heat transfer in the entrance region of rectangular ducts rotating about a parallel axis. Boussinesq approximation is invoked to take into account the centrifugal buoyancy. Mechanisms of the secondary vortex development in ducts of isothermal or iso-flux walls are explored by a vorticity transport analysis. Numerical solutions of the parabolized velocity-vorticity formulation of Navier-Stokes/Boussinesq systems are used to corroborate the above theoretical analysis and provide evidence for the relevance of axial variation of transport rates to the evolution of the secondary vortices. The present results also reveal that the rotational effect in an iso-flux duct is more significant than that in an isothermal flux. In a fully developed flow region, the iso-Bus case can still retain a secondary vortex effect, while the effect vanishes in isothermal ducts.