Self-assembled silver nanoparticles with an average diameter of 5 nm have been successfully fabricated by reduction of Ag+ with ascorbic acid in the mixture of water, alkylamine, and oleic acid. Thermogravimetry (TG), differential scanning calorimetry (DSC), and contact angle measurements indicate that oleic acid molecules are well capped on the silver nanoparticles. The effects of temperature and reaction-medium pH on the morphology and composition of the silver nanoparticles are discussed. A decrease in pH leads to a tendency to produce silver nanorods and nanospheres. The temperature can affect the thickness of the organic layer on the surfaces of the silver nanoparticles. The stabilities of the silver nanoparticles in the nanofluids were monitored at different temperatures. Thermal conductivity enhancements were determined in kerosene-based nanofluids with the prepared silver nanoparticles. The surface-capped silver nanoparticles exhibited excellent dispersity in kerosene and conventional organic solvent such as n-hexane and chloroform. The highly dispersible silver nanoparticles are therefore suitable for the preparation of oil-based nanofluids.