An experiment of natural circulation of supercritical water in parallel channels was performed in bare tubes of inner diameter 7.98 mm and heated length 1.3 m, covering the ranges of pressure of 24.7–25.5 MPa, mass flux of , and heat flux of up to . When the heat flux reached , the outlet water temperature jumped from 325°C to 360°C, associated with a decrease in the flow rate and an initiation of dynamic instability. When the heat flux exceeded , the flow instability was stronger, and the flow rate increased in one channel and decreased in another one. Until the heat flux reached , the outlet water temperatures of two channels reached the pseudocritical point, and the flow rates of two channels tended to close each other. The experiment with a single heated channel was also performed for comparison. The measurements on the heat-transfer coefficients (HTCs) were compared to the calculations by the Bishop et al., Jackson’s, and Mokry et al. correlations, showing different agreements within various conditions.