Abstract: Experiments were carried out in a rectangular loop to study the characteristics of supercritical carbon dioxide natural circulation. The influence of pressure and temperature difference on the mass flow rate was investigated and the effect of geometric of test loop was also preliminarily studied. Results show that no flow instability was found under the experimental condition. There was a peak for mass flow rate. Mass flow increased quickly before the peak point and decreased very slowly after the peak point. The effect of system pressure on mass flow rate was similar to subcritical natural circulation and higher mass flow rate was obtained in higher pressure.Temperature difference had vital effect on mass flow rate. Just small temperature difference between hot leg and cold leg could result in apparent change of mass flow rate. Geometric of test loop had vital effect on the characteristics of supercritical carbon dioxide natural circulation and more in-depth experimental research should be done to study it.