Pt catalyst supported on carbon nano-tube (CNT) was extremely active for the selective oxidation of CO in H-2 at room temperature, which was remarked contrast to the Pt supported on an active carbon (Vulcan carbon) and a graphite powder. Complete oxidation of CO was attained on a 5 wt.% Pt/CNT catalyst (0.8 g) at ca. 40 degrees C when the O-2/CO ratio in a flow of H-2 (20 mL/min) + CO (3.0 mL/min) + O-2 + N-2 was adjusted to be larger than 0.75 at the total flow rate of 100 mL/min. Specific activity of the Pt/CNT catalyst was explained by efficient provision of reactant molecules diffusing on CNT surface to Pt particles.