Cubic nanocrystalline Co:Yb2O3 in the size range of 8-38 nm were synthesized by sol-gel method. Annealing temperature should be higher then 600 degrees C. The morphology and the particle size were controlled using the electronic microscopy (SEM). Magnetic susceptibility measurements of all samples were done in a temperature range 2-50 K. In this region only the lowest Kramer's doublet is populated, and for that level the effective magnetic quantum number is deduced. The Curie-Weiss paramagnetic temperature theta(x) obtained for the low-temperature region depend linearly on the Co3+ concentration. Above 30 K the inverse magnetic susceptibility deviates from linearity due to the crystal field influence. (C) 2010 Elsevier Ltd. All rights reserved.