Abstract- In this paper, two-dimensional natural convection heat transfer in semi ellipse cavities is investigated using lattice Boltzmann method. The Prandtl number is taken as 0.71 that corresponds to that of air. Heat transfer and flow pattern are predicted at various Rayleigh numbers ranging from 104 to 106 for different aspect ratios. By increasing of the aspect ratio, the heat transfer rate in the cavity is increased for low Rayleigh numbers, but it is decreased for high Rayleigh numbers. The obtained results of the lattice Boltzmann method are validated with those presented in the literature and show that the lattice Boltzmann method can simulate heat transfer and flow pattern in complex cavities. Analysis of heat transfer in a semi-ellipse cavity using second order boundary condition on curved surfaces is among the novelties of the present work.