This paper demonstrates that, by using oil-in-water micelles, nonstoichiometric zinc ferrite nanoparticles are fabricated. Control in size and composition is obtained. The nanocrystals are characterized by a spinel structure with two sublattices in tetrahedral and octahedral sites. Repartition of Fe3+ in the two sublattices is disordered. The particles are ferrimagnetic with a noncollinear structure (the magnetic moments are not strictly parallel). This is more pronounced with the smallest particles. Zinc ferrite nanocrystals exhibit unusual magnetic properties. The anisotropy constant, K, of nanocrystals differing by their sizes is determined by Mossbauer spectroscopy and from zero field cooled (ZFC) curves. The field cooled (FC) curves show a peculiar behavior with a decrease in the magnetization at very low temperature. The ZFC curves are highly sensitive to the applied field, indicating that Langevin law cannot be applied for zinc ferrite nanocrystals. These unusual magnetic properties are attributed to the inclusion of nonmagnetic zinc ions in ferrite nanocrystals, which induces a magnetic disorder.