Core(Cr)/shell(γ-Fe2O3) nanoparticles were synthesized by mixing Fe(CO)5 and Cr(CO)6 in the 9:1 ratio. These particles exhibit narrow size distribution with 13.5 nm as mean diameter and uniform spherical shape. The TEM image, which is in good agreement with the synchrotron powder XRD pattern, reveals the heterogeneous nature (core/shell structure). The analysis of the pattern reveals γ-Fe2O3 structure and a metal crystal structure. Mössbauer spectra, which support the superparamagnetic behavior determined by H−M measurement, do not show any traceable amount of Fe0. This suggests that the metal component is Cr. EELS analysis and iron mapping suggest controlled stoichiometry and also confirm a core made of Cr and a shell made of γ-Fe2O3.