We present here the synthesis and structural characterization of new nanocomposites made of spherical magnetic nanoparticles of maghemite (gamma-Fe2O3) dispersed in a polystyrene (PS) matrix The gamma-Fe2O3 nanoparticles, synthesized in aqueous media, were first gently transferred by dialysis in dimethylacetainide (DMAc), a polar solvent which is a good solvent for PS Electrostatic repulsions enable to keep colloidal stability in DMAc The nanocomposites were then processed by a controlled evaporation of DMAc of binary mixtures of gamma-Fe2O3 nanoparticles and PS chains. The size of the nanoparticles ranges from 3 5 to 6 5 1101 and can be changed without any modification of the nanoparticles surface The structural organization of the nanoparticles inside the polymer was determined as a function of the nanoparticles' size It was performed by combining very high resolution SAXS measurements which permit to decrease the nanoparticles content down to very low values ((1), similar to 10(-5)) and TEM microscopy Whatever the size, the nanoparticles are organized with a hierarchical structure that shows that their aggregation has been driven by a two-step process At low spatial scale, dense primary aggregates composed of some tens of nanoparticles are formed whatever (1),,. resulting mg flom the first aggregation step For (1),,,5 > 10-4. these primary aggregates underwent a second augieganon step and ale oi ganized at larger scale in finial aggi (Tat es of finite size of 200 [un of radius, with a dimension of 1 7 The size of the dense pi imary aggregates is almost constant when changing the nanopartieles rachus,1 c. the mean mull citation number of primary aggregates elect eases with an increa,;e of the radius