Stainless steel heat pipe and thermosyphon with 300mm in length, 6mm in outer diameter and 0.5mm thickness have been developed and test. The heat pipe was made of 316 L stainless steel and installed with a 200 screen mesh°Copper wick. The working fluid was water. Heat pipe start-up and thermal°Characteristics were investigated at different orientations with slopes of 0° , 10°, 30°, 90° and heat sink temperature was kept at 30 °C. The power input varied from 10W to 180W. It is shown that they°Can be operated in all test°Conditions except thermosyphon at inclination angle of 0°. The results revealed that thermal resistance decreased as the input heat load increased. The lowest thermal resistance of heat pipe and thermosyphon are 0.172 °C/W and 0.245 °C/W at inclination angle of 90°