The iron oxide nanoparticles garner a great deal of attention due as a promising material in various applications. For instance, they can be used as biomaterials for targeted drug delivery, hyperthermia and thermoablation [1]. Furthermore, iron and iron oxide nanoparticles can be used in electronics to improve the mechanical reliability of the solder joints. It was shown that the minor additions of the metal and oxide nanoparticles into the SAC solder reveal notable enhancement in mechanical properties [2]. In the present study the effect of thermal oxidization on the chemical composition of passivated Fe-nanoparticles has been characterized by TEM, X-ray and BET measurements. It was shown that prior to thermal exposure, the core of Fe-nanoparticle is covered with Fe2O3. The average size of the nanoparticles was found to be 50nm, while the oxide shell was about 5nm. Subsequently, the nanoparticles were thermally treated, namely were capped in a furnace at 2000C and 4000C for 1 hour. The X-ray studies have shown more pronounced peaks corresponding to the Fe-oxide with increasing the temperature. Furthermore, the ratio changes between metal core and oxide shell after thermal treatments were investigated by TEM. In addition, the surface area measurements of the nanoparticles were performed by the BET technique. It was shown that the surface area values decreases by increasing of the thermal treatment temperature.