We present in this work a numerical study of a problem governed by Navier-Stokesequations and heat equation. The mathematical problem under consideration is obtained bymodelling the natural convection of an incompressible fluid, in laminar flow between twohorizontal concentric coaxial cylinders, the temperature of the inner cylinder is supposedto be greater than the outer one. The numerical simulation of the flow is carried out bycollocation-Legendre method. The influence of Prandtl and Rayleigh numbers isinvestigated