In this paper, the thermal performance of thermosyphon with a flat evaporating surface combined with different sizes of micro pillars was investigated by experimental method. Micro pillars with different sizes were fabricated on the surface of the evaporator bottom plate to enhance heat exchange. An experimental platform was designed and constructed to test the heat transfer performance of the thermosyphon. Deionized water was chosen as the working fluid and the condenser section of the thermosyphon was cooled using water cooling. The operating pressure of the thermosyphon was measured by a pressure sensor to obtain the corresponding saturated temperature. The wall temperature of the evaporator section, the operating pressure of the thermosyphon, as well as the temperature and flow rate of the cooling water were detected for different heat flux. The evaporator section wall temperature (ESWT) and evaporator section wall superheat (ESWS) of the thermosyphon with a smooth evaporator bottom plate (THSE) or micro pillars on the evaporator bottom plate(THMPE)were comparatively studied. ESWT and ESWS of the thermosyphon with micro pillars were much lower than those of THSE. Generally, when the micro-pillar width was constant, heat transfer was intensified with increasing micro-pillar height. When the micro-pillar width was 200 mu m, ESWT of the thermosyphon with a micro-pillar height of 200 mu m was 12.1 degrees C higher than that of thermosyphon with a micro-pillar height of 800 mu m at a heat flux of 68.6 W/cm(2). When the micro-pillar heights were 200 mu m and 400 mu m, the variations of micro-pillar width evidently reduced ESWT. However, when the micro-pillar height was 600 mu m and 800 mu m, the variations of micro-pillar width barely affected ESWT due to the capillarity and disturbance of bubbles motion caused by micro-pillar changes. At a smaller micro-pillar width, the thermosyphon with higher micro pillars had lower ESWS. When the heights of micro pillars were 200 mu m and 400 mu m, the increase of micro-pillar width was conducive to reducing ESWS. When the heights of micro pillars were 600 mu m and 800 mu m, a smaller micro-pillar width better reduced ESWS of the thermosyphon. (C) 2019 Published by Elsevier B.V.