CO oxidation on unreconstructed Cu(110) at low temperatures was studied by mass spectrometry, AES, LEED, and HREELS. It was found that catalytic CO oxidation took place at 200-230 K, while the reaction never proceeded above 250 K and stopped in a short time below 180 K, The surface at the end of reaction above 200 K was covered with the (2 x 1)-O phase. It was deduced that the reactive species was as-adsorbed oxygen atoms which were not incorporated in Cu-O chains of the (2 x 1)-O phase readily formed above 250 K. The apparent activation energy for the catalytic CO-O-2 reaction was obtained to be a negative value, -15.8 kJ/mol. profiles of CO2 production at 200-230 K were analyzed according to a kinetic model based on the Langmuir-Hinshelwood mechanism and the competitive growth of the (2 x 1)-O phase. Measured kinetic data were reproduced well by the simulation using this model. From the analysis, the activation energy of the reaction between CO(a) and O(a) was found to be 34.8 kJ/mol, and that of the growth of the (2 x 1)-O phase was found to be 16.8 kJ/mol. The activity of the reactive oxygen species on Cu(110) seemed comparable to those of the oxygen atoms on Pt, Pd, and Rh.