For the first time, polyamide-6 nanoparticles were successfully obtained by anionic polymerization of E-caprolactam in heterophase using the miniemulsion process. It is shown that stability of the monomeric miniemulsion was obtained by dispersing either molten epsilon-caprolactam or epsilon-caprolactam dissolved in a polar solvent. During polymerization only the latter miniemulsion stays stable, and nanometer aggregates of polyamide-6 were formed by precipitation polymerization inside the miniemulsion. The syntheses were carried out with different continuous phases, amounts of surfactant, an initiator, and the monomer. The success of the polymerization and the influence of these parameters on the particle size, morphology, and the molecular weight of polyamide-6 were studied using DLS, viscosimetry, TEM, and FT-IR spectroscopy.