Catalytic oxidation of NO (400 ppm) into NO2 was studied over active carbon fibers at room temperature to trap the unreacted NO as well as the oxidized product in forms of nitric acid or its salts to develop an oxidative removal of NO in the flue gas. The heat treatment of a particular pitch-based active carbon fiber of rather small surface area was found to enhance markedly the activity to allow the conversion of 82% in dry, 65% in 80% relative humidity (rh) and 21% in wet (100% rh) air, respectively, at 25 degrees C by W/F of 1.0 x 10(-2) g min mL(-1). The rate of oxidation was examined by varying humidity to reveal marked decrease of the activity above 80% rh. The strong retardation by humidity of the activity of as-received fiber was moderated by the heat treatment of the fiber at 800 degrees C. The catalytic activities of other pitch- and poly(acrylonitrile)based fibers were also examined in dry and wet air to find their inferior activity in wet air above 80% rh.