Colloidal 5.1 wt% Ru/gamma-Al2O3 catalyst was prepared by a microwave assisted, solvothermal reduction of RuCl3 in ethylene glycol in the presence of gamma-Al2O3. The catalyst subjected to heat-treatment in hydrogen up to 700 degrees C, was characterized by BET, XRD, TEM and H-2 chemisorption. As-prepared catalyst contained Ru nano-particles with mean size of 1.5 nm and narrow size distribution uniformly distributed over the support. The nanoparticles were stable on the alumina to 500 degrees C, but treatment at 600-700 degrees C caused some sintering of Ru due to migration and coalescence of a part of smallest ruthenium nanoparticles. However, even after H-2 treatment at 700 degrees C, large amount of Ru nanoparticles with sizes of 1-3 nm remained in the catalyst. H-2 chemisorption data revealed decrease of Ru dispersion from 0.28 to 0.19 by hydrogen treatment at 700 degrees C and were in good correspondence with TEM results. On the contrary, mean crystallite sizes obtained from XRD were strongly overestimated.