Mixing of two fluids of different densities in a solar storage tank has been simulated by the state of art Lattice Boltzmann Method (LBM). Unlike conventional computational fluid method, there is no need to define an interface shape. In the present simulation with LBM, the interface is determined by the simulation results without any assumption. The present results of mixing of two different fluids can be expanded to multiple components fluids and is expected to provide guidance for chemical fluid mixing in solar storage tanks. Hence to improve the storage capacity and shift the discharge time as demand request.