A numerical study of natural convection from a two dimensional horizontal channel with rectangular heated blocks is performed. Because of the problem periodicity, the studied domain is reduced to a "T" form cavity that presents a symmetry with respect to a vertical axis passing through the middle of the openings. The governing equations are solved using a control volume method, and the SIMPLEC algorithm is used for treatment of the pressure-velocity coupling. Special emphasis is given to detail the effect of the blocks spacing (gap) on the heat transfer and the mass flow rate generated by the natural convection. The results are given for the parameters of control as, Rayleigh number (10(4) <= Ra <= 8 x 10(5)), Prandtl number (Pr = 0.72), opening width (C = l'/H' = 0. 15), blocks gap (0. 15 <= D <= 1.0) and the blocks height (B = 0.5). The results show that the flow structure and the heat transfer depend significantly on. the control parameters. Two principal kinds of problem solution are raised. (c) 2005 Elsevier Ltd. All rights reserved.