In this study, iron oxide (FeOx) nanoparticles were synthesized using iron (Fe) by-products recovered from NdFeB permanent magnet scraps, and the effect of heat-treatment temperature on the physical properties of the FeOx nanoparticles was investigated. In order to prepare the FeOx nanoparticles, 2.0 M ammonia (NH4OH) solution was added to an iron by-product solution diluted to c.a. 10 wt% in D.I. water, which led to the precipitation of the iron oxide precursor. Then, the FeOx nanoparticles were synthesized by heat-treatment at 300 ℃, 400 ℃, 500 ℃ and 600 ℃. After that, the physical properties of the FeOx nanoparticles were investigated in order to understand the effect of the heat-treatment temperature. The results of the X-ray diffraction (XRD) analysis showed that the diffraction peak in accordance with the <104> direction increased as the heat-treatment i ncreased, a nd a d iffraction p eak i ndicating the α-Fe2O3 crystal structure was detected at heat-treatment temperatures above 500 ℃. The BET specific surface area analysis revealed that the specific surface area decreased as the heat-treatment temperature increased to above 400 ℃. Observation with a high resolution transmission electron microscope (HRTEM) showed that rod-shaped nanoparticles were formed, and the size of the nanoparticles showed a tendency to increase as the heat-treatment temperature increased.