We report a new method to prepare magnetite nanoparticles with narrow size distribution and controlled particle size. Magnetite nanoparticles were prepared in the pores of the biopolymer chitosan. To develop pores with different sizes, chitosan was gelificated with several concentrations of the crosslinker glutaraldehyde. X-ray diffraction studies showed pattern characteristic for magnetite particles with diameters varying from 4 to 12.7 nm. Mossbauer spectra at room temperature showed only paramagnetic component for the smaller particles, and magnetic and paramagnetic components for the bigger particles, indicating the transition from superparamagnetic to blocked magnetic regime as the particle size increases. The zero field cooling (ZFC) magnetization measurements showed peak temperatures varying from 55 to 151.4 K. The ZFC peak temperatures showed a trend in agreement with the size of the particles. Considering the different experimental time window between Mossbauer spectroscopy and magnetometry, the results obtained from both techniques are in agreement. (C) 2013 Elsevier B.V. All rights reserved.