**Abstract:**
In the present work, two-phase Rayleigh-Benard problem is simulated by lattice Boltzmann method. Two horizontal layers of immiscible fluid are confined in a rectangular cavity. The vertical walls of the cavity are insulated while the horizontal walls are maintained at different constant temperatures. Two-phase lattice Boltzmann method is used to model hydrodynamic field and a passive scalar approach is implemented to model the thermal field. The viscous heat dissipation and compression work done by pressure are neglected. The present model is validated with the single-phase Rayleigh-Benard problem and good agreement is observed. The applicability of this new lattice Boltzmann model for simulating thermal two-phase problems is the main objective of this study. Furthermore, a comprehensive parametric study of the problem is carried out for wide range of different non-dimensional parameters. It is found that with increase of Rayleigh number, the fluid motion becomes stronger and the isotherms are more distorted. Also with decrease of the ratio of Prandtl number of upper fluid to lower fluid, conduction dominates in the upper layer. It is concluded that this new thermal lattice Boltzmann model has a great capability to model thermal two-phase problems.

**Keywords:**
Thermal lattice Boltzmann method, Two-phase Rayleigh-Benard problem, Passive scalar approach, Rectangular cavity.