Ag-Cu alloy nanoparticles were prepared by the inert gas condensation (IGC) process. X-ray diffraction (XRD) patterns show that particles were phase separated as pure Cu and Ag with some Cu incorporated in the Ag matrix. The particle size obtained either from Scherer's formula or electron microscopy images shows no systematic change of the size of either pure Cu or Ag-Cu particles in the evaporation temperature range between 800 degrees C and 1400 degrees C. By using lattice constant values and Vegard's law, the composition of Cu in Ag particles was calculated to be 6.6 vol pct. Analyses of the alloy nanoparticles suspended in hydrocarbon rotary pump oil were also carried out in order to determine the changes in thermal conductivity and viscosity of nanofluids. Thermal transport measurements have shown that there is a limit to the nanoparticle loading for the enhancement of the thermal conductivity. This maximum value was determined to be 0.006 vol pct Ag-Cu nanoparticles, which led to the enhancement of the thermal conductivity of the pump oil by 33 pct. Beyond this maximum loading, thermal conductivity decreased and reached back to the pure oil thermal conductivity value.