Uniform spherical Co-B amorphous alloy nanoparticles were prepared by ultrasound-assisted reduction of Co(NH3)(6)(2+) with BH4- in aqueous solution, and the particle size was adjusted by changing either the ultrasound power or the ultrasonication time. During liquid-phase cinnamaldehyde (CMA) hydrogenation, the as-prepared Co-B catalyst exhibited much higher activity and better selectivity to cinnamyl alcohol (CMO) than the regular Co-B obtained by direct reduction of Co2+ with BH4-. The higher activity can be attributed to both the higher dispersion of Co active sites (S-Co) and the higher intrinsic activity (R-S). The higher selectivity can be attributed to both the uniform Co-B amorphous alloy particles and the strong electronic interaction between Co and B, which enhances the competitive adsorption of C=O group against C=C group in the CMA molecule. Meanwhile, the stronger adsorption for hydrogen oil Co active sites was more favorable for C=O hydrogenation in comparison with the C=C hydrogenation. (c) 2006 Elsevier Inc. All rights reserved.