For horizontal concentric double rectangular duct with cooled outer duct and heated inner duct, two-dimensional numerical calculations were analyzed. Numerical calculation used repeat calculation tool of "Excel" were carried in a range of Prandtl number Pr=0.7, aspect ratio γ=1.0～3.5 and Rayleigh number Ra≦4060. It was illustrated streamlines of natural convection, isotherms and average Nusselt number. The flow patterns of natural convention consist of a pair of the "L-type" circulation in side-bottom space and small vortices in upper space. Those flow velocity increases when Ra increases. When γ is growing larger, the vortices number increases. It was shown that 1, 2, 3 and 4 pair of the vortices appeared for γ=1.0, 1.5, 2.5 and 3.5, respectively. When γ is growing larger, pattern of isotherm is complicated. The average Nusselt number at the bottom surface decreases with increasing aspect ratio.