Nanosized Janus particles were prepared by ligand exchange reactions of a Langmuir monolayer of hydrophobic alkanethiolate-passivated gold nanoparticles at relatively high surface pressures with hydrophilic thiol derivatives injected into the water subphase. The ligand intercalation between adjacent particles led to impeded interfacial mobility of the particles. Consequently, ligand place-exchange reactions were limited only to the side of the particles facing the water phase, leading to the formation of amphiphilic nanoparticles which exhibited hydrophobic characters on one side and hydrophilic on the other, analogous to the dual-face Roman god, Janus. The unique amphiphilic characters of the Janus particles were confirmed by a variety of experimental measurements, including contact angle measurements, FTIR, UV-visible, and NMR spectroscopies. Interestingly, the Janus particles might be dispersed in water, forming micelle-like aggregates, as revealed in dynamic light scattering and AFM measurements.