A sonication-assisted method to synthesize supported Co/SiO2 nanoparticles was developed using NaBH4 as reducing agent. Co nanoparticles with an average size of 2.4 nm were obtained. Further fine-tuning of the particle size was achieved by adding stabilizer. TEM and XPS confirmed that the dispersion was significantly improved by using sonication. After calcination at 400 oC or 600 oC for 2 h, the size of the Co3O4 nanoparticles was increased to ca. 8 and 11 nm, respectively. Traces of Co2SiO4, B2O3 and Co(B4O7) were also detected after calcination at high temperatures. This method provides a new approach to the controlled synthesis of supported transition metal nanoparticles.