In the present work, Lattice Boltzmann Method (LBM) is used to analyze the natural convection flow and heat transfer in a square cavity with an inside thin heater. The vertical walls of the cavity are adiabatic, while its horizontal ones are maintained at constant temperature. The square cavity contains a thin vertical heated plate, with the temperature. The present problem is governed by the Rayleigh number, the heater length, the heater location, and the Prandtl number,. The results of the study show that the position and the length of the heated plate alter significantly the temperature distribution, the flow field and the heat transfer in the cavity.