This article deals with utilization of blast furnace slag nano-fluids in two-phase closed thermo-syphon heat pipes for enhancing heat transfer at various states of operation. The utilization of nano-fluids obtained from X2O3-, XO-, XO2-, and X2O-type oxides, such as Al2O3, Fe2O3, CaO, SiO2, MgO, MnO, K2O, and Na2O, on the improvement of heat pipe performance has been separately reported in a number of studies in the literature. The present study experimentally demonstrated the effect of using a nano-fluid obtained from blast furnace slag comprised of various types of metal oxides in varying ratios on improving the performance of a heat pipe. The slag was obtained from the iron blast furnace of Karabuk Iron Steel Workings (Turkey). Triton X-100 (Dow Chemical Company) dispersant was used in the study to produce the blast furnace slag/water nano-fluid via direct-synthesis. The 2 wt% concentration of blast furnace slag/water nano-fluid was used as the working fluid in heat pipes. A straight copper tube with an inner diameter of 13mm, outer diameter of 15mm, and length of 1m was used as the heat pipe in the present experimental study. The nano-fluid filled 33.3% (44.2ml) of the volume of the two-phase closed thermo-syphon. Three heating power levels (200, 300, and 400W) were used in the experiments with three different flow rates of cooling water (5, 7.5, and 10g/s) used in the condenser for cooling the system. An increase of 22% was achieved in thermal performance of the two-phase closed thermo-syphon when 2 wt% blast furnace slag containing nano-fluid was used to replace pure water at a heat load of 200W with a cooling water flow rate of 5g/s.