Ni1-xCoxFe2O4 (x=0.6, 0.8 and 0.9) nanoparticles have been synthesized with various crystallite sizes depending on the thermal treatments and composition (cobalt content) using the sol-gel combustion method. The size of nanoparticles has been controlled by thermal treatment. On the other hand, the magnetic property of the ferrite has been controlled by changing the heat treatment. Morphology and particle sizes of Ni1-xCoxFe2O4 have been studied using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The presence of functional group has been identified by Fourier Transform Infrared (FTIR) spectra. From TGA-DTA studies, the weight gains of Ni1-xCoxFe2O4 nanoparticles have been observed and it might be due to capping organic molecules with oxygen at temperatures above 200 °C. Magnetic properties of Ni1-xCoxFe2O4 particles have been analysed using VSM and it is found that saturation magnetization (Ms) has increased with particle size and has coercivity (Hc) increased initially and then decreased. The Ms and Hc values decreased with the increase of content of cobalt in Ni1-xCoxFe2O4. © 2009 Elsevier B.V. All rights reserved.