The gravity heat pipe with small diameter in the startup process was investigated experimentally by analyzing the influences of heat input，inclination angle，cooling water flow rate on the startup properties.The tested gravity heat pipe was fabricated with copper tube and charged with distilled water.The heat pipe has the outer diameter of 8 mm with the thickness of 1mm，and total length of 1500 mm.The results show that with increasing heat inputs，the startup time decreases and the time to reach a steady effective thermal conductivity is shortened.The startup properties at inclination angle of 60° are superior to those at 30° and 90°.The startup temperature difference and startup time at 90° obtain the maximum values.When the cooling water flow rate is 10L·h-1，the startup temperature difference and startup time both present the maximum value，and the effective thermal conductivity demonstrates the shortest time to reach the steady state.The relations of the startup temperature difference and startup time at other cooling water flow rates are all satisfied with the sequence of 10L·h-1 ＞ 40L·h-1＞ 20L·h-1＞ 30L·h-1.