Abstract: This paper presents the experimental study of the pressure drop for low-flow forced circula- tion in medium pressure sub-cooled boiling. The average void fraction is also measured in this experiment. Based on the experimental results, predictive investigation on pressure drop in low-flow sub-cooled boiling is carried out in this paper. The investigation shows that the pressure drops in low-flow sub-cooled boiling are dominated by pressure drop due to static head. Since the pressure drop due to static head are strongly affected by void fraction, it is very important to select the calculation model for void fraction. The Saha model and Levy model cannot be used to determine the void fraction for the calculation of pressure drop reliably. The NVG model and the true mass quality model proposed by the authors can be used to calculate the void frac- tion for the calculation of pressure drop in low-flow sub-cooled boiling correctly. Through the investigation, the proper method for the calculation of pressure drop in steam-water low-flow sub-cooled boiling with forced circulation is suggested in this paper.