Abstract: Theoretical studies on characteristics of high volumetric quality flow of natural circulation under rolling motion were performed. Unlike the fluctuating characteristics of single-phase flow of natural circulation under rolling motion, the high volumetric quality flow doesn’t fluctuate and is relatively stable. A mathematical model was developed to calculate the flow rate of both the single-phase flow and the high volumetric quality flow under rolling motion. The calculated results are in reasonably good agreement with experimental data. Analyses were made to various terms of pressure drops and frictional coefficient under such two types of flow conditions, and the analytical results show that the increase in the pressure driving head and frictional coefficient leads to the relative stable flow characteristics of the natural circulation of high volumetric quality flow under rolling motion.