In this study, the steady-state characteristics of two-phase natural circulation core cooling in advanced boiling water reactors is investigated. The natural circulation is controlled by the mean density difference between the coolant inside and outside of the core. The Boussinesq approximation was applied in the momentum equations for two-phase flow analysis that considers the buoyancy force, which is related with thermal effects. The results obtained in steady state with the buoyancy force in the core were compared with the standard approximation. It was found that the buoyancy effects in two-phase flow can be important in the nuclear design of advanced boiling water reactors, due to the fact that nuclear parameters, such as thermal-hydraulics and neutronic, power, void fraction, fuel temperature, heat flux, mass flow rate in the core (superficial velocities), feedwater flow, and main steam flow, are appreciably influenced by this physical phenomena.