MFe2O4 nanoparticles were obtained in the presence of natural compounds as carboxymethylcellulose (CMC). The CMC polymer had a double function as a capping agent and as a protecting agent in the growth process of nanoparticles. The synthesized nanoparticles were characterized using thermal analysis (TG, DTA, DTG), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and a vibrating sample magnetometer (VSM). The XRD patterns indicate that all the samples were formed in single phase spinel structure. The results also show that the samples calcinated at 500 degrees C for 6 h have the best crystallinity and the calculated crystallite size was in the range of 6-13 nm. The thermal analysis and FTIR spectra indicate a core-shell structure of the MFe2O4 nanoparticles obtained. (C) 2012 Elsevier Ltd. All rights reserved.