This paper describes preliminary findings of Au nanoparticle on glassy carbon (GC) electrodes for electro-oxidation of carbon monoxide in basic conditions. Electrochemical cyclic voltammetric results showed that CO oxidation simultaneously occurred in the anodic and cathodic sweeps during one cycle, and CO electro-oxidation activity was remarkably different in the anode for the different sized Au nanoparticles. The ultrafine catalyst metal particles (2 and 6 nm Au) were more active compared to the larger ones (12, 24 and 41 nm Au). The dependence of the activity on the particle size can be explained in terms of their effect on the number of active sites and the different surface gold oxide species.