Cobalt nanoparticles in the range of 1.7-7 nm were synthesized in water using sodium borohydride as reducing agent and subsequently immobilized on TiO2. Both colloidal and TiO2 supported NPs were characterized by TEM, XRD, HRTEM, XPS,TGA, ICP techniques and their catalytic performance of the supported catalysts evaluated in the Fischer-Tropsch synthesis. As a general trend, an increase in activity and TOF was observed when the particle size decreased. These results were rationalized by the higher reducibility of the catalysts bearing the smaller nanoparticles. The unconventional stability of these relatively small CoNPs in FTS was attributed to the promoting effect of boron on the stability of the cobalt nanocrystals. (C) 2016 Published by Elsevier B.V.