Nickel oxide nanoparticles of different sizes are prepared by a sol-gel method. Samples are characterized by X-ray diffraction and transmission electron microscopy. Dc susceptibility measurements as a function of temperature and field are done for various particle sizes. We find peaks in zero field cooled susceptibility vs. temperature curves. In sufficiently low fields the peak temperature decreases with increasing particle size whereas at higher fields the peak temperature increases with increasing particle size. We propose that these nanoparticles consist of antiferromagnetically aligned core spins and a spin glass like surface layer. (c) 2005 Elsevier B.V All rights reserved.