The flow and heat transfer for two-dimensional horizontal concentric double rectangular duct with cooled outer duct and heated inner duct were analyzed by a finite-difference procedure. Numerical calculation used repeat calculation tool in "Excel" by a range of Pr=0.7, aspect ratio γ=1.0～2.5, Ra≦4060 and inclined angle φ=0～45°. Streamlines of natural convection, isotherms and local Nusselt number Nu were illustrated. It has been found next three results. (1) The flow patterns of natural convection consist of a pair of the "L-type" circulations in side and upper space with small vortices in upper space. (2) The number of small vortices depends on γ, Ra and φ. (3) Isothermal lines and Nu depend on the flow patterns of small vortices and L-type circulations.