An experimental apparatus was constructed to measure the thermal conductivity of a fluid containing nano-sized particles. A transient hot-wire method was used to measure the thermal conductivity of the fluid. In addition, the experimental equipment was manufactured to investigate the heat transfer characteristics of nanofluids. Experimental analyses for the effects of nano-sized Ag particles on heat transfer were performed in double-pipe heat exchanger systems. The following conclusions were drawn from the present experimental studies: The normalized thermal conductivities of water, a water-EG mixture (Conc. of EG = 20%), and water-ammonia mixtures (Conc. of NH3 = 17.5 and 20%) increase as the weight percent of Ag nanoparticles increases. The overall and individual heat transfer coefficients of water containing nano-sized Ag particles increase with the fluid flow rate in the laminar flow region. The overall and individual heat transfer coefficients of water containing nano-sized Ag particles are superior to those of water at the same fluid Reynolds number. The overall heat transfer coefficient of water containing nano-sized Ag particles increases from 5 to 13% and the individual heat transfer coefficient increases from 17 to 25%.