The photodecomposition of propyzamide in the aqueous solution was investigated using TiO2-carbon black composite films. It was found that the photodecomposition rate was largely influenced by the content of carbon black in the photocatalyst film. The decomposition rate increased with increasing the carbon black content up to an optimal value, beyond which the photocatalytic activity showed a decreasing tendency. The increase of the photodecomposition rate seems to result from increase of adsorbability for propyzamide in the TiO2-carbon black composite films. With further increase of carbon black from the optimum value, however, relative occupation of TiO2 to carbon black decreases, and the time needed for adsorbed propyzamide on carbon black to diffuse to TiO2 particles becomes great, resulting in the decrease of the apparent photocatalytic activity of the film. Experiments employing a circulating photoreactor system also showed that the TiO2-carbon black composite films had higher photocatalytic activities than the naked TiO2 film.