In order to more easily separate TiO2 photocatalyst from the treated wastewater, TiO2 photocatalyst is immobilized on coal fly ash by a precipitation method. The titanium hydroxide precipitated on coal fly ash by neutralization of titanium chloride is transformed into titanium dioxide by heat treatment in the temperature range of 300-700 degreesC. The crystalline structure of the titanium dioxide shows anatase type in all range of heat treatment temperature. The crystallite size of anatase increases with increasing heat treatment temperature, with the draw back that the removal ability of NO gas will decrease. When the coal fly ash coated with 10 wt.% of TiO2 is calcined at the temperatures of 300 and 400 degreesC for 2 h, the crystallite sizes of anatase appeared about 9 nm, and the removal rate of NO gas shows 63% and 67.5%, respectively. The major iron oxide, existing in coal fly ash as an impurity, is magnetite (Fe3O4). The phase transformation into hematite (Fe2O3) by heat treatment improves the removal rate of NO gas for TiO2-coated coal fly ash. (C) 2004 Published by Elsevier B.V.