Abstract: Numerical simulations on steady state behavior of supercritical water natural circulation in a rectangular loop were carried out.Different expressions of gravitational pressure drop were evaluated.The calculation results indicated that with the heat load variation,the mass flow rate of the steady-state natural circulation obtains a maximum value when the heater outlet temperature is near the pseudo-critical temperature,and the maximum value decreases with an increase in heater inlet temperature.The density difference and velocity difference between the heater inlet and outlet increase when the temperature of the whole heater moves towards the pseudo-critical area,while the pressure drop decreases in this condition.The evaluation of the gravitational pressure drop expressions indicated that when the expression proposed by Ornatskiy and Razumovskiy was used to calculate the gravitational pressure drop in a relatively long tube（2 meters）,the results were lower than those obtained from integral method which could be considered to be the real gravitational pressure drop,and the maximum error was near 30 percent.On the basis of Simpson integral method,a new method was proposed to calculate the gravitational pressure drop.The new method showed more satisfactory and could be adopted in experimental data analysis.