Loop thermosyphon (LT) is a two-phase closed device that provides an alternative solution to overcome the freezing problem associated with the traditional photovoltaic/thermal (PV/T) system. The overall performance of a LT-PV/T system is significantly affected by volume-filling ratio. To reveal this effect, a prototype of the LT-PV/T system was designed and constructed in this paper; and long-term outdoor tests under 24%, 32%, 40% and 48% volume-filling ratios were carried out. The results show that the daily average photothermal performance of the LT-PV/T system presented a trend of increased first and then decreased with the volume-filling ratio increasing on the selected sample days. The highest performance was generated under the 32% filling condition. The solar evaporator had an even temperature distribution when the volume-filling ratio was no less than 32%; the temperature difference of the solar evaporator decreased with the volume-filling ratio increased, the values were 1.5 degrees C, 1.2 degrees C and 0.6 degrees C, respectively. Performances of the collector and system were linear fitted based on the outdoor tests, the fitting results showed that both of them shared the same variation trend, the optimum volume-filling ratio lied in between 32% and 40% of the whole system volume, but nearer to 32%. (C) 2019 Elsevier Ltd. All rights reserved.