In consideration of the effects of the local resistance and the frictional resistance in the natural circulation loop, the analytical relation between the natural circulation mass flow rate G and the heating power Q can be express as G similar to Q(m) , m=f(R-e,R-n), where the R-n, is the ratio of the local pressure loss coefficient and the friction coefficient. The relation of flow-resistance to the natural circulation mass flow rate can be expressed as Delta p(f) similar to G(r), r = f(t, m). These correlations is tested with experimental data from the experimental facility of HRTL-200 II and showed reasonable agreement. (C) 2016 Elsevier Ltd. All rights reserved.