In this paper, stable metallic copper nanoparticles protected by bis(ethylhexyl)hydrogen phosphate (HDEHP) were prepared in the organic phase. In the synthesis of copper nanoparticles, the bis(ethylhexyl)hydrogen phosphate (HDEHP) acted as both a phase-transfer agent and a particle protector. It was also found that the protective ability of HDEHP was different with different diluent polarities. Despite zero valent Cu initially forming in all the solvents studied, it has been found that the zero valent Cu can easily transform into oxides in those solvents with high dipole moments under ambient conditions. Only in those nonpolar solvents, the metallic copper nanoparticles are stable in air. In addition, theoretical analyses were made in order to interpret the interaction mode between the copper nanoparticles and the surfactant HDEHP.