This work was designed to examine the effect of various thermosyphon diameters and working fluids on the thermosyphon performance. A thermosyphon made from copper tubes with three different external diameters 7, 13 and 22mm with thickness of 1mm is used in this work. The length of evaporator and condenser were 120 and 300 mm respectively. Working fluids were water, acetone and Freon R11 working fluids are tested. The obtained results have shown that the temperature gradient was decreased when increasing the thermosyphon diameter. Also, the results showed that the Merit for water had the highest value than the other working fluid at the operating range of temperature. The results were shown the heat dissipation by the thermosyphon increased when increasing the thermosyphon diameter for all working fluids. Also the heat dissipation from the thermosyphon with working fluid of water had a highest value of heat dissipation than the others working fluids. The percentage decreased in the temperature gradient by using thermosyphon with diameter of 22mm for water, acetone and Freon R11 were 73.53 %, 68,53 % and 52.35 % respectively compared with that without using thermosyphon.