With the protection of sodium oleate, Ag nanoparticles are produced through the reduction of AgNO3 with NaBH4 in an aqueous solution. The possible mechanism of phase transfer of the Ag nanoparticles was discussed. At a suitable concentration of sodium oleate, after adding NaH2PO4, the oleic acid molecule can change its position on the surface of Ag nanoparticles under the effects of water and toluene and become amphipathic. So most of the nanoparticles form a film between water/toluene. For the case of a higher concentration of sodium oleate, excess sodium oleate will form a closed monolayer film on the surface of the Ag nanoparticles. After adding NaH2PO4, the oleic acid molecule cannot move on the Ag nanoparticles surface, thus the colloid particles are hydrophobic but not amphipathic. So most of the particles transfer to the organic phase. UV-Vis spectra, TEM and conventional metallographic microscopy are used to characterize the Ag nanoparticles and nanoparticles films. (C) 2002 Elsevier Science B.V. All rights reserved.