One of the important use of heat pipes is the thermal control of satellites. The performance characteristic of a wickless copper heat pipe was investigating experimentally. For that aspect, a test rig was built up for a heat pipe with 1.4 m length, 18 mm internal diameter and 2 mm thickness charged with R134 a as working fluid with a temperature range from -40 °C to 110 °C. The evaporator section was electrically heated with heat flux varying from 200 to 600 kW/m2. The condenser section was water cooled by using an overflow tank. Thermocouples were embedded on outside surface along the length of the heat pipe, and this surface was thermally insulated. The heat pipe transient responses as well as the axial distribution of wall temperature were plotted, for the three sections of the heat pipe, at different values of heat flux. The overall thermal conductance and the heat transfer coefficients at the evaporator and the condenser sections were plotted against the heat flux. These values were compared with the results published in literatures. The effect of some parameters on the heat pipe performance had been studied like the tilt angle and the adiabatic length of the pipe. The results show that the increase of the tilt angle or the adiabatic length of the pipe reduces its heat transport capacity. The correlation of Sakr and Ramadan [ 8 ] for the entrainment limit of heat pipes, had different values of n for R11, R12, R22 and R118 as working fluids. Applying the present results in this correlation, it had been found that n= 4.056 for wickless heat pipe with R134a.