Free cooling based on thermosyphon is a promising technology for energy-saving in data centers. Integrated system of mechanical refrigeration and thermosyphon does not need auxiliary refrigeration system, therefore it is an ideal way of free cooling based on thermosyphon. To study its free cooling performance and potential, it is built that a distributed-parameter model of the thermosyphon mode in an integrated system of mechanical refrigeration and thermosyphon and validated by experimental data. The simulation results show that the cooling capacity and circulation flow rate increase with the increase of height difference mainly due to higher driving force of gravity, while the increase is little when the height difference is higher than 0.5 m. The cooling capacity and circulation flow rate decrease with the increase of connection pipe length due to higher flow resistance, therefore the connection pipe should be as short as possible. The cooling capacity increases rapidly with the temperature difference and reaches 5.3 kW when the temperature difference is 15 degrees C, with an EER of 14.3. Also with the performance model and weather data of China, annual free cooling potential in China and corresponding energy-saving and economic benefits are analyzed. For the studied cities in China except those in the hot summer & warm winter zone, the free cooling percentages are approximately 30-70%, the annual energy-saving rates are 16-49% and the payback period is 1.7-4.3 years. Therefore, free cooling based on integrated system of mechanical refrigeration and thermosyphon has great application potential for energy-saving of data centers. (C) 2016 Elsevier Ltd. All rights reserved.