In this paper, numerical simulation of natural convection in a closure square cavity with heating source on the basement is carried out using thermal lattice Boltzmann method. The purpose of this work is to analysis the physics of mass and heat transfer in such a condition. For four different Rayleigh numbers (Ra=10^3~10^6 ), the relative heating area ratio S is changed in the range of 0.1-1.0, correspondingly. It is found that the flow pattern is symmetrical for Ra increases from 10^3 to 10^6, and the boundary layers at low temperature become thinner, also the circulation rate is higher. For a special Ra, Nu is as a monotone increment function of S, and the value of which almost increases linearly when when S is below 0.8, and then reach to its maximum exponentially.